Thanks to all for the inputs.

However almost all are focused on diagnosing the 'cause'. As I stated in the beginning I know the cause...the oil flow through the 3x18 "700HP" oil cooler is too slow for 625hp because of the current bearing clearance restricting that flow. Someone else very knowledgeable in custom engines had my same oil temp runaway problem during extended WOT running and tried all the things suggested herein and a whole lot more over a period of years on a PAIR of closed cooling Whippled Merc 496HO engines with the exact same 3x18 "700HP" oil cooler that I have. When the oil flow rate through the oil cooler was increased by looser bearing clearances and a higher GPM oil pump to keep the oil pressure up with the looser bearing clearances the oil then stayed cool for unlimited WOT running (650 - 700 HP).

I am however looking for a work around to provide additional oil cooling for extended WOT running w/o engine rebuild/redesign. Please limit further comments to that focus.

Thanks

As said by others, I would change the cooler out to get rid of the thermostat. If that doesnt fix the issue then you can put it back on. Your oil bypass in the filter pad is plugged isnt it? otherwise the hot oil is just being put right back into the pan.

You might think about running a less viscous motor oil. 10w-30 synthetic. See what you get. It will change the low temp flow characteristics of the system and keep more oil going through the cooler, less bypassing. If it helps it will validate your engine is set up tight.

Facts:
You know what the problem is.
You know what the fix is.
You can't bandaid a boat.
If it's worth fixing, its worth fixing right--best fact I learned from my Father when I was 10.

You aren't boating right now anyway. Pull the motor, fix the bearings. Be happy.

That is an interesting idea. Thanks.

Never seen a crankcase cooler like you describe and a quick google search didn't find much. Not sure how well something strapped to the bottom/sides of the pan would work, guess it depends on if the pans are steel or aluminum, what does the cooling tubes look like? How are they attached to the pan to achieve the best heat transfer characteristics.

It definetly sounds like you have done your home work with regards to trying different things. I am not going try and suggest how to fix you "plumbing" problem but I am curious about a couple things

1. What is your typical water inlet temp?
2. What size lines are you running for your water system?
3. Where is the cooler in the water plumbing? Directly after the sea pump?
4. What kind of sea pump do you have?

The thermostat design actually allows some of the engine oil to bypass when oil is <190F with the cooling section always open and cooling the oil flow. Then the thermostat closes at 190F to shut off that oil bypass of the cooling section to then force all oil through the cooler. The center filter pad oil bypass is plugged by the filter pad bolt. The offset filter pad oil bypass was upgraded from the stock 11 psi valve to the 25 psi valve (supplied by Raylar) which only opens to bypass the oil filter and oil cooler section if the oil flow resistance in that system exceeds 25 psi. Since oil pressure at the oil pump (before the oil filter and oil cooler section) is the same as the oil pressure at the cam bearings and mains there is essentially zero pressure drop across that 25 psi bypass valve so it should not be a factor. It will be plugged when the engine is refreshed.

I have made WOT runs that resulted in the 270F oil temp in winter (sea water temp in the 40's, air temp in the 30's) and summer (sea water temps in the 90's). It makes no noticeable difference except maybe the oil gets hot a little faster in the summer.

My base engine is a Merc 496HO still with the stock closed cooling heat exchanger and plumbing except the stock 160F coolant thermostat replaced with a 120F thermostat and the Dana exhaust cooling water fitting/hose adapters. The drive is an X1 with the lower water p/u plus the side p/u's producing 40+psi sea water pressure at WOT. The cooler is #2 from the sea water pump behind the #1 power steering pump fluid cooler only.

The crankcase pan is cast aluminum with a fairly flat bottom section. I am envisioning something that looks like a typical add on truck transmission oil cooler that would fit there (aluminum cooling line coils with aluminum fins) held tight contact to the bottom flat section of the crankcase (cooler fin edges pressed against/contacting the crankcase bottom) by tension springs and an aluminum plate on the opposite side of the cooler fins from the crankcase. Conductivity between the cooler fins and the crankcase could be improved with a coating of white lead. The sea water to this crankcase cooler could be tapped from the line between the p/s cooler and the oil cooler and dumped separate. I tested a 5/8" dump line at that point before for additional drive shower cooling (drive also has WOT lube overheat issues) and the sea water flow was still sufficient for the other coolers.

This might be a stupid question, but could a bad oil filter be causing the flow problem? Collapsed internally, or just poor quality?

This has been going on for years and many oil filters including tests of very high flow low restriction low filtration oil filters.