Shiny pistons!
You touched on quite a few subjects to comment on.
First, what are you using to determine coolant temp? The dash gauge is an “indicator”, but rarely reads correctly. An aftermarket gauge works much better, with mechanical gauge working better than electric (in my opinion).
I was cruising eBay not long ago, and ran across this and this might also be a great tool (if it will fit), maybe.
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Note the max reading on last cap. It shows up to 400' F/200' C - dang that's hot.
I am not saying you have an overheating concern – all I’m saying is I would not go by the dash temperature gauge reading as your only indicator.
Second, any evidence of coolant in the oil or valley area?
A person might have to drain engine oil to make for sure.
Stock small block Mopars are not known to have a lot of head gasket problems. It happens, but not a real common thing.
Two things come to mind at the location indicated. One is leakage from intake gasket (which is a lot more common problem I see).
The other issue is block or head flatness at that location. You mentioned “everything was flat”, I’m guessing you checked both the cylinder head and cylinder block using an official straight edge and feeler gauge? Most people and machine shops will check cylinder head for flatness – but it is uncommon for shops or people to check the cylinder block for flatness (not saying it wasn’t checked – just covering my bases).
The same thing goes for cylinder head intake surface and intake needs to be checked for flatness – especially aluminum cylinder heads or aluminum intakes.
No matter what surface it is, it is hard to seal an un-flat surface with a flat gasket (or a flat surface with an un-flat (not parallel) gasket).
If assembled – the best way to check for coolant leaks (hot or cold) is with a cooling system pressure tester. In some cases, even white paper towels placed against key areas is helpful.
Lastly, I’m not fond of “stop leak” especially for cooling systems - and can not recommend using it. The substance gets into unintended locations and stops flow, like heater cores, or in small holes drilled in picture (red circles). Once you flush out the coolant (which a person needs to do every 2 years) then the “stop leak” has been removed and leak reappears. If you have a coolant leak, it is best to just find and fix it (IMO).
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That said, it can be a “wonder drug” for used cars (good or bad, depending if you are seller or buyer).
BudW