I would look at doing 1/4" steel plate similar to what brick masons use to re-inforce them along the length of the step. If you have to go more than few inches, you might as well go the entire way. You will probably have to go thicker than the standard tread thickness eventhough ipe is quite hard. Keep in mind the additional weight of the steel plate as well as the ipe tread as ipe and steel weight a lot when its that thick. Which leads me to the next thing - I'd consider a lot more than just 'blocking' to hold a freeform tread. My guess is that the weight of each treat could be 150lbs but thats not the big issue - its the dynamic load as people are bounding up the stairs. . . You may want to look at 2x12 continuous along the run of stairs or something sturdier than that. Ipe is available in larger pieces as the tree is quite large. You'll just have to find a good specialty lumber supplier that can get it for you. I've seen curved handrails cut out of ipe, so its definitely available. It's just very hard and very heavy.

We have 5" walnut floors and dark-stained oak treads on our stairs. I don't think they have to match.

We have been crazy busy this last week removing all the steel studs, furnace and two 50 gal water heaters from the basement to make ready for the new concrete floor so sorry I am just now getting back to here. We just finished late yesterday and we are both really sore from all that manual labor getting everything out. The company that will be putting in the concrete started at 7 a.m. this morning welding the iron that will hold the corrugated steel sheets that will hold the 4" concrete floor. When they are done with that, we will spend the next three days laying in the vapor barrier to cover the dirt and we also have to put extensions on the two sump pits. Prior to this, we had to have the OSB flooring removed and about 1 foot X 2,200 square feet of expansive soil removed, new helical piers drilled, the second sump pit installed and new perimeter trenches dug and new french drains and rock put in so this has been a horrific costly project. We have done any and all work we can do ourselves to defray costs. Thank you all for so much helpful advice...I appreciate all of it!! We did check into having tubes put into the concrete to carry heated water to heat the floors but about 80% of the floor will be carpeted and we have been told that heated floors would not be efficient with carpet. There may be some great pad to use under the carpet that provides more insulation too. There will be a tiled bath and a work/hobby room that we will put the heated mats under the tile. We may put in a high efficiency stove/fireplace in the future. I have been reading about all kinds of concrete sealers, one has tea tree oil in it which protects against mold and it seals against radon too so I think we will seal the concrete floors for an extra layer of security! We will also have this super heavy duty vapor barrier down on the soil. We live in Colorado so it can get very cold here and we also have radon issues. Back to the stair issue...I have received about 50 responses to the ad I put on Craig's List. One guy said he was experienced with stairs and it was probably a half day job and would be about $100 labor plus materials. That sounds really low but I plan to call each one back and screen them - or at least the ones who answered my ad in the way I asked them to - and the ones that look good I will have come look at the stairs and give me a bid. I realized that it doesn't make much sense to have them look at it until the concrete floor is in which will now be the second week of June.

OK, finally took some pics of the stringer and transition to baseboard in hall at top of stairs - think this will work? Lovely gap - over 1" from wall to edge of stringer Colonial "stop" molding (not thick enough so have to lay flat instead of laying it against wall - my cousin was going to finish off the end with a mitred return, just tape holding it right now) But that still leaves the problem of transitioning to the thinner, lower baseboard, and a gap b/t the stringer and the wall So I'm thinking of running the stop molding vertically as well - he'll just have to figure out the angle to join the 2 pieces of stop (and of course cut the baseboard so it fits snugly against it). Either that, or just do the mitred return and then put a block in between like crazyhouse6 did? Which do you think will look better? I want to run the stop vertically at the bottom of the stringer too, to hide the end grain and cover the awkward chipped corner bead on the drywall where it meets the stringer just to the side of the newels (and not do the molding where DH is holding it here, just run the oak board to "box" in the bottom of the newel higher than the stringer), so would it be best to do the same treatment top and bottom? Sorry, I can get DH to hold the newel and take some better pics of the bottom tonight while I hold the stop over the end grain.