This is when you start practicing the speech for the ol lady on why you need heads, stroker kit, etc. Ive got it down pat

Yup

Sounds like a bad case of Covid 19.

I've always done the leak-down with piston at TDC compression stroke.

That's the right way.. If you get it perfectly at TDC, the eng won't rotate but that can be difficult to do. Usually takes me a couple of tries. There's a replaceable orifice between the 2 gauges of the differential press tester The size of the orifice is determined by the eng size. If it's not the right size orifice, the differential readings will be off.

However, assuming the compression is a bit low due to bottom end leakage, I would think it would still produce some boost at 4k RPM. As mentioned prev, I'd try a new blower belt before tearing into the eng or blower..

I put a scope in the cylinders to see the pistons. Didn't see any holes but they did seem carboned up. Did the overheat cause the rings to stick?

Put a new blower belt on it and go run it and report back.
If pistons are have significant carbon build up, its from a rich condition. That rich condition could be from dumping in fuel at WOT and not making the rpm to use the fuel due to lack of boost.

Check or replace your temperature sensors
Did you replace the thermostat ?

1) Engine Coolant Temperature Gauge Sender

2) Water Temperature Gauge Sender

The overheat condition can cause the rings to lose their tension. If you are truly seeing 40-50% leakage past the rings, they have lost their seal. Pull it out and tear it down.

Any update OP?