Thought I'd share the results of installing a pair of RM Revolution Marine exhaust manifolds. Boat is an '87 Baja Sport 220. As far as I know the engine is factory stock 330HP 454. Bravo 1 swinging a box stock 26 pitch Bravo prop. Intentionally propped to bottom of RPM range so I could see the gains and not get into an over RPM situation. With the old cast iron echo chambers small outlet manifolds it would run 66mph @ 4200rpm. The swap to the RM manifolds dropped 65lbs. After the swap it runs 4400rpm and 68mph. Same GPS app. Same stretch of lake. Water is probably 15° warmer for the second test session. Air temps about the same. Both tests were on water that was the next thing to glass. Same amount of fuel. Did everything to keep it as much the same as I could.
Next is a single plane Merc intake modified so the runners match the peanut port heads. Probably a waste of time. But experimenting is fun and I don't have the dough for new heads at the moment.

Is it a Gen 6 block?
Try to find some 279 heads, bump in compression and they have the large oval ports.

No, it's a good ol Mark IV.
I'll look into going backwards with those heads. I know you can move the old heads forward to V & VI.

You can usually find some 781 heads pretty cheaply as well, but without a domed piston you're staying at 8:1.
Pretty impressive how much exhaust makes a difference. I picked up 10 mph on a 7.4 MPI with cam, intake, and exhaust, guess the exhaust played a larger role than I thought.

After looking into it the Gen VI heads are a bolt on so long as the Gen VI head gasket is used. The hunt begins. Thanks for the idea.