Transmission Help

[jhiggy94]

I have tried searching the forums but i couldnt find much info on this. I am working on my 72 K20 and my th350 trans went out recently. I would like to just take it out and take it to a transmission shop and have it rebuilt. BUT.. Right now the plan is to just keep the original 350 engine in the truck but eventually i will repower either to a ls engine or to a diesel. (right now i dont have the money to do the swap) I would like to know if I built up the th350 would it still survive behind a newer ls or a 6-bt cummins? If it wont what would be my options right now? I know the TH-400 is a more heavy duty and that wouldnt be a bad swap, if i built that up would it do better? What other options do i have?
Thanks a lot for any help