I know the mounts for the motors themselves are usable on a big or a small block. How ever is the base assembly mount (the one bolted into the boat) for a small block able to be used with a big block?

Yes for all chevy engines

Same bolt pattern for the coupler housing, but Bravo coupler has a larger opening than one for an Alpha.

mounts in same place bbc and small block. moved back a bit for v6.

are you sure about that,i think the small block mounts a little farther back than the big block.

This one I am sure of. I have switched out a small block for a big block on two boats with my father (he was the knowledge and I was the free wrench). But since he passed away I am trying to do things with a combination of memory, and you guys.

they put mount bosses on big blocks a little farther south on the block so both engines could sit on the same frame mounts with flywheel in the same place in the car or truck. I put a 454 in an old formula I pulled 350 out of. mount sat on same holes on stringer. only thing I had to change was the power steering-alternator mount...and the exhausts. it was over 10 years ago. I am gonna go measure both blocks and double check though. I have a 454 and a buncha small blocks in the garage.