I use a Fein Multimaster with a flush cut E blade. Dremel also has a much cheaper version called a Multi-Max. Lay down a metal speed square where you want to make a cut. Tape it down good using blue tape and then make your cut using the fat part of the speed square as your guide. Cut back slightly at an angle so the new board will drop in clean.

In the old days they scrubbed floors regularly with lye and other harsh cleaners. Daily cleaning involved strewing sand on them and sweeping it away. Many old houses had no subfloor. I certainly don't have any except where additional flooring was laid later. Although I have plastered ceilings in my first floor, many early homes had the planks of the second story floor visible from the rooms in the first floor. Going back to my comment above, if you can see ground through the cracks, you really should consider laying down some vapor barrier (on the surface of the soil). Unless you live in the desert, your building is taking up huge amounts of ground moisture and that will impact your comfort and the health of the house structure, in a big way. However, for floor finishes, there are a number of ways you can handle it without using poly, or stain. The simplest way is to clean them well (that's scrubbing on your hands and knees) followed by carefully drying them with rags. The next step is to remove any stains you want to shift (water, cat pee, etc.) This will raise the grain a bit, but it can be smoothed down by light hand sanding without impacting the established patina. Then you can seal the floors with a variety of things from tung oil to varnish to shellac. All of the above have pros and cons, but each is an historic finish. One of the advantages of doing this type of floor work is that it is relatively reversible (oil is less so, see below) and can be patched up in sections where necessary because of later wear, unlike poly finishes. Another advantage is that you may be able to avoid having the floors sanded, which will not only save you money, but preserve the thickness of the floor boards (they can only be machine sanded a few times before they must be replaced) and you will not lose the patina of old wood. You would be surprised how often people arrive here and report they are paying extra to install floors with artificially created patina. My personal choice is a pure tung oil finish followed by a coat of Butcher's wax. This is quite durable and easy to maintain. And if one of my cats has an accident the fluid stays on top for a bit and I can clean it up with no problems. (Cat pee, or any fluid, not cleaned up promptly will create a mark on almost any type of wood floor finish, including poly after it has had some use.) One thing to think about with tung oil finishes is that if you do choose that, you will have to sand the floors down before switching poly at some later date. This is somewhat true even for shellac and varnish which can get caught in cracks and interfere with the smooth flow and adherence of later poly coats. Keep in mind, too, that some "tung oil" finishes have only a small amount of tung oil and they are mostly polyurethane varnishes. I prefer pure tung oil. (You can read about it at the Real Milk Paint company website.) Some people who are allergic to nuts (peanuts, cashews, etc.) may also be allergic to tung oil which comes from a tropical tree. I'm not sure how much of an issue it is on floors, but it can be problematic to use it on kitchen counters, or at least I've seen reports of that being a concern. As for getting the smell of old cat pee out of unfinished wood: it's hard to do. The best product I've tried is something called Get Serious. It's for sale at PetSmart places and online. IMO it works better than the more commonly used Nature's Miracle. YMMV. One other caveat, I've seen reports that washing unfinished wood with Murphy's Oil Soap can lead to subsequent finish adhesion problems. I don't use it, so I'm not sure. However, FWIW, I once toured Montpelier in VA , the home of President James Madison, and I was lucky enough to be the only visitor that day so I got a behind the scenes tour of the restoration work in progress. And the highly skilled restorers were washing the floors with Murphy's. I live in a modest country-style Greek Revival farmhouse and I have great appreciation for simple, plain, buildings. Too often the grander houses are the ones that one sees or reads about. It warms my heart to hear about other plain houses getting the care they deserve as they tell a story of their own. You can't really go wrong trying to keep what you can of the old materials; repair rather than replace; and go slow so you don't wind up doing things that shouldn't have been done. If these 19th c buildings weren't solidly, if simply, built they wouldn't have survived into their third century. I try very hard to make sure that whatever I do is along the lines of careful maintenance of what I have, and when necessary, minimal changes at the same level of build-quality. I sure don't want future owners in 2108 shaking their heads at the penny-foolish and thoughtless "improvements" done under my care. Molly~

Solid wood paneling is a classic, very valuable and cannot be replaced. I'd do something to keep that. I agree that the original aesthetic of the wood was to add warmth with all the natural hard materials like stone, slate, iron, etc. Depends on how much of a purist you want to be with the aesthetic. The original design is flexible as to what colors and stains would work. You could paint too. The only real crime would be to tear out something that is as valuable as real wood paneling. It was even expensive back in the day when it was installed. It has value because it is high quality and beautiful. I would seek to do something that retains the pluses. If you want to maintain your house as an homage to the middle of the last century, then obviously "restore" rather than "redo." Oh, and just a ponder, is that two story fireplace material original? Or was it originally brick too? Love your house BTW! Very elegant.

@Garry Crabbs II - the 3/8" thickness is going to make this tricky. First off, MEASURE the amount of wood ABOVE the tongue. You can find a floor vent and pop it out to view the cross section of the wood. Measure the wood ABOVE the tongue. It MUST BE 3mm (1/8") or MORE to get a SINGLE sand/refinish event completed. The photos show you are not "almost down" to bare wood, you are AT bare wood. That means this floor "missed" it's sand/refinish date by 5-10 years. And we're assuming that is oil based urethane. That was VERY common in the 60's. Nothing special about it. You will need to sand this 3 separate times (each "pass" takes off about 0.5mm of wood which is why you NEED 3mm to start with). You will start with a low grit paper....like 60 grit. You will need a palm sander with the same grit to get into corners and close to walls. Then you will move up to your mid-range grit (80 grit is the next step up from 60). Again cutting in the walls and corners using the palm sander and the same grit level as the big machine). And then your final pass (assuming everything has gone swimmingly) you will finish with the highest grit level (100 grit is the next step when using the 60-80-100 grit system) doing the walls and corners once again with the palm sander. Some guys work with the 60-80-100 grit system. Others prefer the 80-100-120 grit system. Plan on purchasing MULTIPLE discs (assuming you use a drum sander) in each grit level. Some refinishers use 4-5 "papers" per grit level each time they do a floor. That means they use as many as 15 discs/pads for each home! Just be aware of that. You might want to find out which system will leave most of the dark stains that have penetrated the raw wood....and which system leaves the MOST amount of wood on your floor. That is rather important. You don't have a lot of wood to work with so you want to make sure you have something left over that you can live with for the next 20 years. Once the sanding is done, you chose a stain (if this is DIY I suggest you SKIP this stage) and then apply 3 coats of finish over the floor.