What you have in your pics is a factory finished floor. It is difficult to get that kind of consistent look when staining unfinished, especially if the unfinished has boards with mixed sawing characteristics. The face sawn material will take up and hold the least pigment and will be the yellow boards you are seeing. We can sometimes get a more even look by using a sand grit sequence to allow more pigment to be deposited on the floor and by doing what we call waterpopping; you may still get variations, but it will look as even as it's going to get. I don't do this...but some of my colleagues will first use an analine dye and then, when it is dry, will follow that up with a pigmented stain. Talk with your finisher.

We have used Mullican, Oakcrest, Turman, Sheoga and a few more that I can not remember at the moment. They're all great with quality unfinished products. Sheoga has beautifully milled flooring, however, they will not sell direct to consumer. Mills can typically get any type of hardwood from whichever region. Speak with someone and let them know you would like the Northern oak and not a southern variety. Oh, I almost forgot. You can not simply ask for Red Oak. There are a few different grades of hardwood. Select, No 1 Common, No 2 Common and No 3 Common(No 3 is sometimes referred as character/cabin grade.)

Those "gaps" are mild. They are normal. They are expected. They tell the story that they are original. They are meant to be there. The new floors will eventually do that too...which means they will match (eventually). It might take a decade...but it will happen. And don't forget that once everything is sanded down, you will be VERY surprised at how those "gaps" seem to go away (visually). Why, you ask? Because some of that is dirt. Once the floor has been sanded down (1-2mm of wood is taken off with the sanding), the dark material between the planks gets shaved off as well. And voila...they look cleaner (because they are). Relax about the absolutely normal spacing between planks. Unless you have museum quality climate control, this LITTLE bit of shrinkage is normal and to be expected with every floor (especially if it is 30 years old).

I'm in Canada, so I get the cool feeling. As a cork flooring professional I have 'issues' with in-floor radiant heat because the product I LOVE (love cork....LOOOOOOOVE IT!) cannot go over top. Cork has TOO HIGH of an insulation value. It counter-acts any heat that is trying to push through. Wood is a semi-conductor. It will allow 'some' energy (ie. heat, electricity) through while blocking the rest of it. For that reason, most people who want in-floor radiant heat to warm their toes use tile, stone or carpet (are you trying to heat the HOUSE with the radiant heat or are you warming your toes?). The other issue I have with in-floor radiant heat = it severely limits the product you are allowed to use over top. The BIGGEST issue is with ELECTRIC HEAT. Sigh....the stuff that is used most often in 'renovations' with radiant heat. Ahem....electric radiant systems are not the one's who tell you what can and cannot go over their product. They will 'claim' just about anything to get you to purchase their product. Nope. It is the FLOORING manufacturer who says, "Appropriate for use with hydronic in-floor radiant heat." Notice I changed to 'hydronic' with that last statement? Yep. Hydronic = water/oil/gel. It is the most efficient, expensive and the most rigorous to install. It can (potentially) raise the floor height of the room by more than 1". It can be hung from the subfloor BELOW the floors (that means you need access to ceiling in your basement (assuming you have a basement...I can't tell). That will keep the floor-raise to a minimum (or zero...). It is still expensive to install. So...my question is: which TYPE of in-floor radiant heat are you using? Hydronic? Electric? Or my favourite 'oldy but a goodie' = Forced Air (yes...I've seen it! Soooo interesting...from a house built in the 1950's)? The TYPE of in-floor radiant system AND the REASON for the heat (whole home system or just warming your toes) will dictate WHAT you are allowed to put over top.