6 speeds (and 5 speeds with TY754XXXXX - from 98-00 models) have 8 bolt bell housings and the others have 4. They still bolt up fine, you just dont use all the holes.
As for the gearbox mounts - they look identical, and will (and do) work. Maybe the 6 speed mount is a bit tougher though due to the extra weight of the box? (a subaru techy person could probably answer that for you).
The 5 speed gearbox subrame is also identical to the 6 speed, so everything bolts straight into a GC or Legacy.