Carbs were not Quick Fuels. I'm not happy with the carbs but I don't believe they are involved here. Except as it might relate to the engine problems, I'm going to leave the carbs out or now.

No rings were stuck. Trying to figure out if the bore clearance issues were caused by heat or whether the heat just contributed to the issue.

What blocks and bore size?

4.5 gen 6

heat..possibly..any oil left in something out of the engine ? have it tested will tell you alot

How long did this run hot for? It looks like a heat seizure to me and that could also explain the detonation you see on a couple of plugs, especially if this thing ran hot for a bit before you noticed it. Do you have any pistons that are not damaged that you could measure to see what skirt clearance was?

Pistons look like a clearance issue to me. It's pretty well understood in the marine industry that you add a little more piston/bore clearance for the hot piston/cold water condition a lot of boats see (without enclosed cooling systems). From other things I see, this engine builder may not have done that, may not have know that. If the cyls were out of round and tapered, then there may have been ring sealing issues allowing oil contamination in the chambers which will almost assuredly lead to detonation. The valve train is a no brainer, but I will say that's why I'm not a fan of the Jesel Sportsman shaft rockers because of this potential. They really offer nothing over a good stud mount system with a girdle. If you want shaft rockers, get something with a one piece intake stand.
And I will NEVER touch the face of a bearing with anything other than a quick wipe with white ScotchBrite but that's just me.

Water temp never moved. Oil temp on only one engine went up to about 240. Maybe a minute or so at that temp

Yeah I know it's water under the bridge, but it would still be nice to know. Engines are at shop and completely torn down. Machining etc already done. Bob at MK is not a builder, he will be supplying the cam if the engine builder is cool with his cam specs because ultimately he's responsible for them. The only parts actually ruined are the pistons. The heads and valves checked out ok despite the fact that they were installed as received. I think I'll go a little more aggressive with the cam this time as the engines are in a different boat than they were originally spec'd for. The carbs were gone through by a guy in Niagara Falls so they should be good to go. I'll dyno with both my current 4150 and a Quick Fuel 4500.

You know bad rigging can kill motors also :-)