71 Demon

Hello all! I have a 71 Demon and am trying to get it together! It was a 6 cylinder car originally and the guy put a small block K member in it! It did not have an engine or transmission in it when I bought it! I am rebuilding an old 318 engine that I had in the garage and plan to put it in the car. I have a rebuilt 727 transmission that I got from a friend, don't know what it came out of, and was wandering if it would mount up! The bell housing is correct and was concerned with trans mounts to the car. Which I will have to buy! Also I was wandering about length. Was there differences in length between A, B, and E body 727's. I found an old discussion where one person said that there was no difference in length for 727's in all cars Mopar made. Just trying to verify? Hoping to use what I have as the budget is very limited! This is my first project car and trying to avoid blunders! Any and all input is welcome! Oh and I will be looking for some front seats as well!