Looking at going with the 315cc heads with all the great responses-
planning on running on a 502 and then upgrading to 540 in the future (once in larger boat). Do not want to purchase two sets of heads. Would rather sacrifice a little on the 502 for a better mild 540 build.

As far as Inconel exhaust valves-

Do you recommend the AFR BBC 1.880 Tulip Inconel Exhaust Valve 7633 valves?
or is there some other exhaust valves I should run?

You won't be giving anything up on the 502 build either IMO

Call Bob for the best price on those AFR's . He will give you the lowdown as far as valve and spring upgrades . Marine Kinetics Ph# 5856548583

I put ferrea Inconel in these new heads I'm building, not cheap about 38 bucks a piece

I will definitely give Bob a call.

Last question- thoughts on the AFR Adjustable Guide Plates?
prefer the adjustable or solid valve guides?

I was surprised to see the price of these heads wow. I can buy a 500 horse motor for just a little more lol

But that 5 might be a 6 with the right heads .

Go with the adjustable guide plates. One of the most important parts of reliability to your build will be valvetrain geometry and stability.

I'm starting with a 425 (ish), and did the math and was pretty surprised by $3500 heads not including inconel, tax and shipping plus a cam plus install wow.
Horsepower isn't cheap

if you buy your afr heads from bob madera you will get inconal exhaust valves,the correct springs for your application,the right size for your engine,and at a very good price.and it will be a lot less than 3500.bucks.