Misfire at cruise, popping/afterfire at 3000RPM

Righty Tighty said
So it was likely at 45*BTDC when my initial was at 20, am I following here?

I must be misisng something. How you're getting 45*?
I got 41 by adding the 4 degrees difference between the initial of 20 and the initial of 16 to the 37* you measured at 2800 rpm.
In doing so I assumed the 16 and the 20 degree initial was measured at the same rpm. That's admittedly a fairly big assumption.

Righty Tighty said
And with the vacuum advance connected, likely even higher (assuming the vac advance is functioning)?

No. With your foot bringing the throttle 3/4 to fully open there isn't enough vacuum.
Vacuum advance is for light to moderate throttle lean conditions.

Righty Tighty said
I have an FBO limiter plate kicking around, thought about installing that. BUT, I think I'd rather troubleshoot and find the actual problem first.

Good thinking.

Righty Tighty said
I'm definitely worried about causing damage, so when it starts to stumble and act up, I immediately let off the throttle.

Great!