If you are bolting up a 5.0 roller block to a C4, it's pretty easy. You have to make sure you get the correct 50-Oz flexplate and corresponding balacer. You can pick from a 3 bolt retro version or the standard 4 bolt version of those. Avoid the 1979-1985 blocks as the quality control and block lightening makes these junk.
The 1986 and newer blocks have thicker decks, thicker cylinder walls, and semi-siamesed cylinders at the bottom of the bores.
Since those blocks don't have the cast in provision for the mechanical clutch linkage, I heard there is an adapter you can get to allow the use of the older style linkage.
Basically 85-92 motors had forged internals from the factory. The late 92s went to a hyper piston instead.
If you don't care about EFI, 85 and up is a good choice, if you want EFI you want to go 89 (88 for California) and newer as these will be mass air instead of speed density. The 94-95 EFI is different and less popular. If you go EFI I would go mass air not speed density, at least if you are going to use Ford stuff.
The 94 and 95 Mustangs got the T-Bird/Cougar intake system because the original Mustang intake would not fit under the lowered hood line of the new car.