Engine Swaps

Other than size, what makes an engine not compatible with a particular car? Why couldn't a buick 455 be dropped into a v6 mustang?

It could be. It's just a matter of how much time and effort that would be necessary to fabricate mounts, brackets, wiring, linkage and everything else needed to mate an engine that is completely different. Most people think it's more trouble than it's worth and prefer to go with something at least a little closer to what the manufacture offered. Still if your around long enough you will see some wild and wacky stuff. Like the Corvette I remember with a Ford motor!

its easier to do "family swaps" (engines from same company or similar, example: oldsmobile to buick) because in older cars (before 80) most car companies made the engine compartments larger so they can fit bigger motors from there other larger cars in to them (chevy vega originally with 4 banger swaped to a chevy 350cid v8 or a dodge dart originally with a 283 v8 swaped to a 440 v8, fits with no problem) but other swaps from outside the family (chevy 350 into a ford falcon for example) can be done with a bit of work. but normally family swaps are easier because parts from some cars can be used without modification. i learned all of that from a engine swaping book from the late '70's acually it acually tells you how you can swap engines and stuff step by step for some vehicles (firebirds, vegas, jaguar x types (V8 swap), 50's ford f100)