C 20 want to install L 96

[Arnulfo]

Greeting from beautiful LOS ANGELES CALIFORNIA. I have a 1970 C20 CST that needs a new motor. The one it has in it now is a non original 350 . Transmission is original. I want to install a L 96. Will it bolt up to the original Th400 transmission? Eventually i hope to install a newer transmission possibly a 6L90E
if it will bolt up without too much hassel. This is my work truck. I do not want to have to cut or notch the frame if i can avoid it. I have never done an ls swap but i have no doubt that i can do it. Just need a little guidance on issues that may arise . I have been wrenching on my own cars and cycles since i was 16. About to turn 60 in january. This is a great site. People here seem laid back and more willing to help than criticize. I hope to contribute by starting a thread when i do the swap. It wont be for awhile though because right now im in the middle of repairing the crumbling foundation to my home. Thank you in advance for any and all help.
Arnulfo