5.3/4l80e help

[volleyballer13]

Been lurking on here for a while trying to figure out what to do. I recently purchased a 1970 Blazer, factory 2wd, with the 307/350 combo. Attached a couple pics of how I bought it and how it looks now. I just purchased a 5.3 out of a 2005 Tahoe that had a 4l60 behind it. I didn't get the trans with the engine. I have a build 4l80 from another project that I would like to use behind the 5.3. My question is how difficult to put it with the 5.3 in the blazer? What do I need? Or should I go with a 4l60 since that is what was behind the motor originally? I have been searching, but get conflicting info or info that is about swapping from a 60 to an 80 and didn't know if that would apply since neither trans has been in the blazer. I know I may have to get my driveshaft shortened, and need to get the computer reprogrammed. All the stuff to do with converting to EFI, I understand, it is just about the trans. Any and all help would be greatly appreciated.