another 360/408 over heating

A great read.
Lots of minds working together.
All banging away narrowing down every possibility.
A couple things. The chemical test proved positive for combustion gases. Not necessarily an issue as it will detect a minute amount of gases.
Pressure test, sure. It holds pressure.
I see it has a heater,and hoses connected. Is heater core flowing correctly? Hasnt been mentioned.
A dead sure way to check for leakage is an airlift.
It will pull air in under vacuum better than water will push out under a pressure test.

Personally, if it doesent boil over, would quit poking at it..
New engine install in a corvette had me panicking until i discovered fans dont come on until 230 degrees. Engineered that way from factory.

Engine running hot? Fans running? Place hand in a safe area downwind of cooling fans,
If heat is being carried out by air moving it will feel like a blast of hot air from your heater ducts.

Not all water pumps are that efficient either. A little too much clearance between vanes and housing and there goes your flow.

I come from a place where heat issues are a lack of heat, working towards a hotter running engine to improve comfort level in the dead of winter.
Running a 205 thermostat in summer isnt a great idea,as it will really push the limit of a pressurized system.