Need input from big cam users.

I doubt the problem has anything to do with the torque converter stall speed. It would have to be one hell of a tight converter to keep it from idling. The converter only knows RPM vs Load, so 750 RPM in a stock engine seems the same to it as 750 RPM in a cammed up engine.
I think you need to verify the valves are sealing correctly & that it's not misfiring 1st. If you have mechanical trouble (like valves) you'll never get the mixture right no matter what you do. If you can, do a leakdown test rather than just a compression test on it because it is way more accurate & you can identify if it's an intake or exhaust valve that's leaking, or rings too. Compression gauges just read peak pressures & won't really show a bleed off in cylinder pressure. An easy way to do it is to remove the rocker assembly from the head(thus closing all the valves on that head) & then blow the air in & check it. That way you can move from one cylinder to the next on that side without having to put each cyl on TDC since you are checking the valves more than the rings. OR you can leave everything in tact & check them as you check your valve clearances cylinder by cylinder. Listen closely for air hissing out the intake or exhaust.
To answer your other question about why valve lash differs so much between cams, it's because of the lobe ramp designs on solids. Hydraulics go with zero lash because you are pre-loading the hydraulic lifter some so there wouldn't be any lash.
Good luck! I hope you find the gremlin soon.