Lots of guys have put rack and pinion on a Stovebolt, but most of them have done it as part of a Mustang II IFS conversion, which of course is an entirely different ball game.
I don't think you'll find a lot of people offering details on putting it on a straight axle, not because I don't like the idea, but because very few people have actually done it.
If Seon has you really should try to find him and ask about his experience.
If you choose wheels and tires with an eye toward saving some weight, I guess I'd believe you might be able to keep the unsprung no worse than where it started.
You might do a little experiment and add 25 poungs to the axle and then drive over some speed bumps and pot holes and then try a couple of the hardest corners you normally use, and decide if you find the ride acceptable. If so, then you're probably going to be fine.
I'm happy to engage in a dialog regarding the project, but I've not done it, and am not likely ever to try it myself...
I will offer one more opinion... Putting rack and pinion on the frame on a straight axle is going to create a serious bump steer problem.
Think about the movement of the tie rods as the axle moves up and down on each end. You'd need to mount the rack at the same height as the axle and even then I think it would bump steer pretty bad.
