I've got both, never compared them because I never had any intention of combining them. If the bodies are similar size, the thickness of the AMT body compared to the newer Revell body is likely a factor. I did an underbody swap (Revell '69 Camaro into MPC '69 Firebird); the bodies are sized similarly. Once I trimmed the Revell bits here and there to account for the differences in body panel thickness, they fit together just fine.
Meanwhile, the Revell Torino GT/Cobra guts into an MPC '71 Cyclone body is one of the easiest deals out there...bodies are similar thickness. The Torino engine compartment walls, once trimmed out of the body, fit like everything was made to go together...
--Mark