M8 vs M6
 

Pros and cons, in conjuction with 1350 hp on pump gas, but could go higher with e85.

The 8 is shorter by a couple inches, that's probably the biggest thing to consider if going on a boat that's already rigged.

Going from trips to twins stagger. So new transom anyway.
What about issues with either? How much hp is lost through the drive?

edit...
For my hp, looks as if m8 is the only way to go.