I have a completely rebuilt 1978 blazer from the frame down. I also have a 1970 C-10 Short Bed 2wd truck. I would like to combine the two and and make the 1970 C-10 a 4x4. Is it easier to swap the suspension or the bodies (or will the bodies even interchange?) or for that matter, with the suspension of the 78 Blazer ever fit the C-10 truck?

The bodies will not interchange. They are two different generations, not to mention the frame of a Blazer is significanty shorter than a short-box truck.

If you don't mind fabbing a little, you just use the body mounts for the c10 and put them where you need them on the blazer frame, and bob the bed on the c10, Had a buddy in HS that put a 53 chevy on a blazer frame. Not too hard.

Could I just extend the frame and drive-shaft etc.

Bob the bed and make it a shorter bed :)

Could I just extend the frame and drive-shaft etc.
yes
Bob the bed and make it a shorter bed :)

but this is better