Re: 1958 Lark 35

[Don]

Are all the thermostat components in place, including the spring loaded valve underneath the thermostat? If that valve is missing or not operating properly, water will bypass the powerhead and the engine will overheat. Seems like you have substituted gearcases and most water pump parts only to get the same result.
Test tanks can be our worst enemies sometimes, although you mention the gearcase is buried deep in the tank. Is the tank clean? Maybe leaves/debris in the tank is clogging the water pick up. It is always worth trying the rig in the actual water to make sure this is not a tank issue.
You mention swapping many of the water pump pieces, but not the aluminum impeller housing, perhaps the aluminum housing is grooved/worn. There is a bleed hole in the impeller housing that must be clear also. You mention trying two impellers, I’m assuming at least one is relatively new. Are you using OEM water pump parts or aftermarket? Several members have reported problems with aftermarket impellers.
Have you had the thermostat off? What is the history of this engine, is it new to you? Is the head/block discolored from overheating?
Is this a fresh or salt water engine?
Finally, you mention resealing the gearcase and that you "haven’t got any water in the oil". A leaky gearcase won’t create overheating problems, I’m not sure if that is what you were asking about though. You have swapped gearcase housings, so I am assuming the SS impeller plate is sealing properly to the gearcase housing. Stripped/broken impeller housing screws, or damaged/corroded casting under the SS impeller plate will create and exhaust leaking into the pump causing overheat problems.
Hopefully some of my thoughts/questions will point out a problem that is quickly resolved for you. Feel free to post pictures if that will help clarify questions/issues.