I want to agree with Tom here. I understand the whole point WAS to use your existing set-up, that you have invested much time into already, but again, unless i read it wrong, it sounds like it started out as a more drag-oriented project (rear set-up) and now you are trying to make it do everything. It also sounds like you created it from scratch, again if i read that right. That would mean that you dont necessarily know if it will even work. Its one thing to make something go straight and work well enough, but another to make it go around a course (or canyon) and handle safely, let alone quickly. I'd just be concerned about spending close to a grand on custom rollers to find out they're still not going to work. At least if you fabbed up a wider (stockish) axle, used some easy to source rollers, and it didn't work, you'd only be out the rear set up. As in, if you HAD to go back to a stock rear set-up, you could still use whatever used stuff (wheels/tires) you found for the rear.

Just an opinion from a guy that is truly loathe to waste even a few dollars needlessly.