We have put the 1500RWD running gear into a 1500FWD just to allow us to have overdrive. Everything is there and at least you dont have to swap the dash like you would in a toledo! Just take the donar car's tunnel aswell and you have all you need.
We rallied a 2 door toledo with 1850 running gear. Del Lines aparantly did that in period, before upgrading to sprint. Our sprint version was pranged before making it to a rally
I think you would find it hard to show they rallied a 1500 with sprint gear, but unless you start winning everything I doubt people will question it. After killing the 2 door, we thought about using a 4 door toledo, but were told we would need to prove it was done in period (I shouldnt have asked!)
If you look at the "sprint" rally car, although it is badged dolomite it has toledo short boot, and flat dash... something a bit fishy there i think!
If you are just doing road rallies - like we did, then i think you are okay up to 74 for the historic class. pre-60 is best - extra points, but really limited choice of cars! 72 is nice as you have a tax exempt rally car when it is not being used.
David