That's true about motor mounts, bellhousing, crossmembers, etc. My 408 is going in a 73 torino sport, which had a 400 has an option, so mounts aren't a problem. I found a c4 bellhousing from a 351m, so I can still use the c4 that was behind the cleveland. I've got a couple sets of 4V headers for a cleveland in a torino chassis, so we'll see how much modification has to be done to get them to work.
73 Gran Torino Sport:351C 4VCC/.030/KB flattop hypers/Comp XE274H/HS 1.73 RR/Torker/750VS/C4/3500stall/TCI manual valve body/9"tracloc 3.50