Differences among years...? Which engines fit?

[Big-T]

I'm trying to figure out the various years of the early Mustangs. Am I correct in understanding that the 64-1/2' to 66' Mustangs are largely the same (in terms of overall body structure, etc.)? From 67' and on they are slightly larger, right?

Will a 351W fit in a 66' and earlier without modifications? I assume they go right into a 67' and later model without problem.

I am thinking of getting a late 60's (67' to 70') fastback for a project car. Are the fastbacks relatively speaking rare?

What exists in terms of aftermarket suspension systems that go beyond just improving the existing suspension? For example, I've seen subframes for early Camaros that take Corvette suspensions, does anything similar exist for the Mustangs? The point being - can it be upgraded with some modern suspension parts to improve handling? I'm drawn to a classic Mustang with a 'modern' handling and performance.

Any thoughts?

T.