Heads?
 

Whats better, I keep hearing about rectangular and oval port heads. Which is better?

Really need more info on what the motor is and what it is to used for. Operating RPM and HP expectations.

Just a general question but since you brought it up....

4.3l V6, gona build to a 9:1cr, somewhere around 300hp hopefully (before the supercharger of course ;) )

Big blocks came from the factory with oval or rectangular port heads. As far as the "modern" factory heads, the rectangulars are higher flowing, but the ports are fairly large (too large for a <450 hp engine). The recent "peanut port" ovals are too restrictive for a >400 hp engine. Aftermarket supplies oval port heads that outperform factory oval or rectangular heads by a large margin.

A GM 4.3 only has one or two flavors of heads to my knowledge. The early 4.3 head and the later 4.3 head. As far as that goes, I think the later head is the same as the early one except larger valve sizes.

I'd expect that you can fit smallblock valves into the 4.3 heads, but you'd have to check to be sure (Dennis??).

300hp is pushing it for a marine 4.3. You can get there, but you'll be running pretty hot cam and may have no usable torque to get the boat on plane efficiently.

Definitely look towards a blower for across the board power.

Course your cheapest way out is still a 350 smallblock.