First, will the tail shafts interchange?
Second, if they will, does anyone have one to spare or swap?

 

a 700R4 is a 4L60. Same thing, different nomeclature. If you're meaning a 4L60E, then it would depend on the era. The early units are the same 4-bolt tail housing, the later units with removable bell housing have a 6-bolt tail housing that is completely different.

 

bolt tail

It's the 4 bolt housing....now if i could just find one from a 3rd gen my day would be good.

 

try craigslist.