BB
Ok, you have a flow bench for a radiator but your pressure tester is
busted.

I'd test the "new" thermostat first.
Then check the radiator again.
The brown goo is a dead giveaway.
Is the radiator original?
I'll bet the half the tubes are plugged, and not allowing the coolant to
circulate properly.

Now what do you want to do with the pressure tester anyway?
Fnd a leak somewhere?
This is all that your going to do with that test anyhow.
It sure won't tell you anything about overheating.
90 psi in one hole doesn't necessarily mean a blown head gasket.
The engine could just have a stuck ring, or ovaled out hole, bad valve
seat, bad valve.
If it had a compression leak 9 times outa ten you'd see tiny bubbles in
the expansion tank.
You have that?
If the radiator is original, it would almost have to be the place I'd
put my money on.

RC