The distance between frame rails and inner fender panels are the same on all FMJ's. The distance from engine to firewall and distance of firewall to core support is the same. The only thing I can think of that would any different between the FMJ bodies (related to the powertrain) might be the distance from engine to hood – and that difference (if any) would be only by a minimal amount.
The only difference on the overall structure of all FMJ’s (other than the outer sheetmetal and other minor details) is some J’s (not all) have a different design transmission crossmember.
Also, all J’s do not use bumper shocks and the heavy bumpers that F and M’s use.
That is why FMJ vehicles are all lumped together is because of the common chassis, powertrain and suspension (and so forth).
If a person currently has any of the three bodies (F, M or J), you can keep engine (with or without transmission) attached to the K-frame (note: exhaust manifolds/headers remain attached to engine but exhaust will need to be detached behind that), detach hoses wiring and so forth, remove the upper control arm brackets from K-frame and drop the K-frame/engine down (or raise body up) and then roll that powertrain/K-frame assembly underneath another FMJ body and attach it - without any difficulty. This doesn’t matter if /6, small block, big block, Hemi (Gen 1, 2 or 3). If you got it to fit in one FMJ, the it will also fit into any other FMJ.
Some things that might need to be swapped out is: throttle cables, brake booster or possibly some core/radiator support (depending if it has the wide or narrow radiator option).
Installing the engine for first time is the more difficult part. Moving an existing engine into another FMJ should be about a day’s job.
BudW