i would check piston to valve clearance just to be safe.never assume anything is what it should be when installing new pistons and or camshaft.i have seen parts with incorrect part numbers on the package,had custom pistons made incorrectly,etc.

As said before, ck piston to valve clearance, think it is ok, but make sure, will go along with valves seized in head and hit, now if the cam timing was off, either changed or screwy timing chain could cause this as well. Also if the timing was real late and exhaust valves overheated in a new build with tight clearances also could have been the cause, make sure balancer has not slipped in rubber throwing timing marks off, found that a few times recently...one killed a complete engine.

Tinkerer got it. Never had that happen but you'd think heads refreshed back to specs that wouldn't be the case but wonder what pushrods and lifters look like. As mentioned for exhaust to have a clearance issue is not a result of this. Pistons/deck height wouldn't be an issue or would have been caught during assm and also would have happen first trip out. If valveguides were clearanced properly I would have to think it would be a lubrication issue or lack of.

Regardless it sucks but also as mentioned this could have been a complete disaster. Now it's getting to the real cause and move forward.

Thank you to everyone who chimed in to help me figure this out, after all the feedback it seems that the most likely culprit is the two valve stems... thanks again everyone

16 valve stems.

You learned a lesson and it didn't cost you that much.
NEVER use an automotive engine shop to rebuild a marine engine.
Cars don't go through the load that a boat does. Heck most cars run at 1500 to 2000 Rpm at highway speeds. With a boat your not even on plane yet at that RPM.
Take your family car and run it up to 5000 RPM for 10 to 20 minutes like some of us do ( that is my fast cruise RPM ) and see how long it lasts.
When building a marine engine you have to be very precise if you want it to stay together. The more HP you want to build the more anal you have to be.
I had my schooling of hard knocks years ago. I have about $20,000 in parts and machining in my 598 - I don't even want to know what it would have cost to have one of the high performance engine builders put it together.
With that much $ invested I don't want to get schooled.
Steve

Those valves hit hard. I'm with tinkerer, guides too tight or lifters. Doubt it's the lifters though. Cam timing or piston to valve clearance causing it to hit that hard would at least have witness marks on the other 6.

Clearance and likely cool oil temps. Not a good combo.

Did the engine builder give you a build sheet? If so would love to see it. Need to know, are the valves seized in the head or were they difficult to remove in disassembly? Based on pic #1 the valve is bent but looks to be closed. I realize this is my first post but I have a background in building and maintaining performance engines. You need to find the cause or causes of why this happened. Don't just fix whats broke without knowing why or you will be doing this again. Glad to help if I can.

Update.....
 

ok so here is an update on the head issue on my newly built motor, it seems the issue was a combination of valve guides being too tight and replacing my exhaust valves with non Inconel valves, the Machine shop ignored my request of keeping everything to standard OEM hp500 specs

i had had the bent exhaust valves tested and they came back 50% chrome , 50% steel ferrous basically a magnetic cheaper grade stainless certainly not Inconel

i decided to order the Ch454a EQ heads and having them gone through at a new machine shop, hopefully my luck will be better this time around