Transmission selection troubles

[hossnat]

Hello. I have a 68 C 10 that originally had a strait 6 and three on tree. I am putting in a big block and auto. Question is what auto to use? I think im set on a 400 but just dont know whether long tail or short. I have to consider the driveshaft loop and whether to use a two piece shaft or not. Want to keep it looking as stock as possible. Going to use original 12 bolt rear. Any 2cents would be greatly appreciated!!