"Marine head? You mean cylinde
"Marine head? You mean cylinder heads?
Not going to do anything for you. The stock cylinder heads on a 307 will breathe fine at boat engine rpm's.
Coupling bolt pattern is the same on any Chevrolet V8 (283, 305, 307, 327, 350) made up to about 1987.
The new Chevrolet crate engines made in Mexico also continue to use the pre-1987 bolt patern.
A GM Crate 350/290HP would be your cheapest power upgrade. It makes 260HP in a boat.
Too bad you rebuilt the 307, but if you could demonstrate it before pulling from the boat, you could probably sell it easily enough.
Then, to use the additional power you would have to change the propeller to a higher pitch , or...you may need different outdrive gearing. Depends what pitch you have now. "
Factory Manual Best, Seloc 2nd best, Clymer if you're desperate.