The radiator brackets may be shaped differently on the 6-cyl cars to allow for the extra length needed for the engine. If you swap to a V8, you will have a mile of empty space between the radiator and fan blade. This can be fixed by getting a fan blade spacer or by swapping to a V8 radiator.
The 6-cyl clutch, flywheel and bell-housing will need to be swapped with Standard trans parts that fit the back of the 390 and allow the starter motor to mount and mate with the ring-gear. But the factory parts, I believe should be a direct bolt-up to the existing gearbox you have.
There will need to be some modifications done for gas pedal-to-carb linkage. Might be easier to scrap all the old mechanical bell-cranks and linkage and swap to a new gas pedal, cable and bracket that holds the cable to the engine block.