How fast can a stock leaf rearend be?

How fast you can go on leafs isn't really the question you should be asking. The answer only depends on how much engine you have and how much tire spin you are willing to put up with.

What you should be asking is how consistent are leaf springs at any given performance level and the answer is, it depends on the tire and the track. My Duster at 10.50's on our home track with all the CalTrac goodies and 11.5x29.5 slicks was inconsistent in the late rounds after the dew point was reached or if it was boiling hot in the afternoon. It ran high 1.30 to low 1.40, 60 foot times. Plus there is also some ET loss with a leaf spring suspension over a ladder bar or four link, even if they hook equally well.

For your application though you can still do a lot, larger tires are the cheapest and easiest route to more traction. Remember, taller tires hook better than just wider tires because as the tire plants it spreads out lengthwise too. You have a cool looking car, I really like it. Good luck with it.