Call Stainless marine, I bet they can configure their manifolds to work in your situation.

I was on the phone with them last week and they were very helpful.

The cavity through the transom assembly will be the next issue. Can only flow so much through there. Not sure where it's limit is but looking at it, it can't be much higher than the manifolds.

I wouldn't really bother with looking at cams any bigger than what's in the mag motors if your leaving exhaust going through the prop hub.

As a matter of fact most manufacturers will state that free flowing thru transom exhaust is required once you get into their performance camshafts.

Ever think about mufflers on thru transom?

I have Lightning Headers with Gibson mufflers, the sound is nice and mellow and I'm pushing 700hp.