Why the hell would you want to put the engine of a late 70's car into a 60's mustang????
I guess you don't realize but most engines in the 70's were absolutely gutless since they started decreasing HP after 1970 to comply w/ the over the top emission controls/requirements, and 2 gas shortages.
I mean I've got a 1972 Mach 1 and in 72 it's 351 Cleveland only put out 170-180HP stock, the best was for that year was the HO 351 w/ 270, so these were far from the powerplants they used to be.
(and as said it only got worse from there)
Besides 60's mustangs are becoming very collectable/desirable these days, even in this economy...I would think to get the maximum value out of the car you'd want to keep it stock, or at least have an engine that was available for the car that same year.
Which engine and transmission is in each car now? The 68's pretty much have space for any engine with the right oil pan and accessories. Granadas are often used for front disc brake parts for the early stangs so be sure and grab those parts before it gets scrapped.