I swapped my '73, 6-3/4" out for a '78, 8". When I got the Mustang II 8" I was very glad I opted for the "Drum to Drum" purchase (it was only $10 more). I found out that though the brake diameter on the friction area is the same, the center hole for the 6-3/4" was too small to fit the 8". So, maybe things changed at some point but in my case the 6-3/4" rear drums did not fit the 8" rear. It would seem logical that the other way around it would work - sort of. That being the 8" drum would fit on the 6-3/4" rear, but the center hole would be too large and would rely on the sloppier lug bolts for centering. More than likely that would cause an eccentric action and pulsating brakes. As it was I welded the 6-3/4" rear to the donor Turbo Coupe when it was hauled off. So, I had to use one of the 6-3/4" drums with the parking brake anyway.
Regarding the drive shaft some setups work and others don't. In general I believe there is a 1/2" to 1" difference. I was fortunate. I went from a C-4/6-3/4" rear to a T-5/8" rear and the original shaft fit perfectly.