Will a rear diff from 2nd gen fit a first gen,with any alterations?

 

The Dana 70 will bolt right it. Dana 80 has thicker (4") tubes and a longer pinion.

Do note, it's about 5" wider, and the spring perches are made for 3" wide leafs instead of 2.5". I have a Dana 80 in my w250 now. (Took it a little further than minimally necessary)
https://www.ramchargercentral.com/threads/dana-80-swap-westcoast-style.211738/