Just a PSA, I am rebuilding my Superboat's 454 and did some reading of old posts. It had some GM 990 iron heads that were going to need some attention so I decided to go to aluminum. I am no expert by any means but wanted to do the build as value minded as possible. I was worried about going with preassembled heads not designed for marine use for obvious reasons.
I called Promaxx and they do set up for several reputable marine builders. They are building me some 317's with PAC springs, Inconel exhaust valves, and appropriate clearanced guides, assembled for $2600.
I went with the 317's instead of the 290's because I needed the bigger chambers and the rest of my junk is set up for rectangular ports. I know the 290's are probably the better choice for most people.
I just thought I would pass this along as it seems to be an excellent value.

I did something similar. I picked up the 290 heads and the Promaxx folks put the inconel exhaust valves and also a spring I needed based on my cam specs. They also gave me a decent deal for getting two sets at one time. Very good people to work with. Those heads have a good air flow properties for the money as well. Still working on my engine builds so still anxiously awaiting to see what the dyno shows.

Your situation sounds just like mine did. 990 heads in bad shape and opted for Promaxx 317 heads on my mild 502 I had built.

I would ask what clearances they (Promaxx) are using for the marine application and see what the guys on here think. My builder opted for the bare heads and chose to do his own set up.

Only reason I suggest this is because I have read a few posts where guys had heads come from AFR or some other head manufacturer set up for marine use and had issues. The consensus has always been never trust the product right out of the box. Verify your clearances prior to installation. Coil bind or stuck valves can be costly and prevented by inspection.

Good luck with your build.

Thanks I was concerned about the valve guide clearance. I was thinking about taking the springs off and checking the guides and maybe testing the springs as well. I’ll do this and report back. I know all it takes is someone making a small mistake to cause big problems.
What type of intake carb cam,etc. did you go with?

Intake

https://billmitchellproducts.com/pro...ere-more-info/

Carb

https://www.holley.com/products/disc...18422862e267bc

Cam

Comp Cams did a Crane 139021 on a Gen V/VI core.

https://www.summitracing.com/int/par...39021/overview

Lifters

Crane 26535-16

https://www.summitracing.com/int/par...make/chevrolet

Rockers

Crane Gold roller rockers

https://www.summitracing.com/int/par...50-16/overview

Distributor

MSD Billet

https://www.summitracing.com/parts/MSD-8560

Exhaust

CMI Fusion headers

https://custommarine.com/product/e-top-header-13910/

Piston JE with Total Seal rings, rods Mercury Racing H beams (Manley), crank OEM

Promaxx 317 heads with HD SS intake valves and inconel exhaust. Isky springs and retainers

How was the valve job from Promaxx with the new valves?

Most aftermaket heads need a going over, and at least a confirmation before install.

Not bad considering they're $1500+ bare.

The 317s flow really well and have a stable port. The 290s aren't as good in that regard. Despite being a bit smaller. A well matched cam and intake will minimize the issue.

I had Saris Performance Marine build and setup some PROMAXX for my engine build, they started with the bare casting and set them to marine spec using high quality components. Engines on the dyno this week.

Post some photos Jim!

Bang for the buck they are some of the best heads out there. Jason and his team at promaxx (in alabama) are also great people. They also do awesome repairs on all makes of head. I have race teams with more than one championship on promaxx heads. If you arent buying dart, brodix, or profiler, buy promaxx and be happy.