A lot depends on you engine use (how long, how hard, under what conditions, etc.);and, it depends on how stiff the engine is. I would guess that the engine would tend to move less at the shaft (near the bottom) than at the top. So an external location that is lower would experience less displacement when the engine starts or vibrates. The vibration is dependant, aside from startup, on rpms, angle of heel, loading, -- assuming a proper alignment and prop pitch.

Clearly, if you have to put the compressor on a bulkhead, it should be on it's own pulley. I also think that you should NOT have the belt particularly tight.

Why are you changing from the Sea Frost compressor?