Slant Six Won't Rev over 3500 RPM

Sounds like a bad fuel pump to me. Even brand new ones fail. Wrong spring installed, diaphragm not operating properly from sitting in a warehouse for years, ect. The pump will basically go into valve float (cavitation)at a certain rpm, and that's it...no more fuel. When you're engine is in neutral there's no load and requires less fuel. If you've got another one laying around swap it on to rule it out. The adjustments you made to the carb and the results you got also support this.

I think you can rule out ignition. There would be a bunch of other symptoms. Valve lash would be heat related not rpm, and I would expect some backfiring, glowing manifolds, or other hoopla. Exhaust is possible and easy to rule out. I'm definitely leaning towards a fuel supply problem. Fuel pump, regulator, carb, ect. Sometimes it's easier (and faster) to swap on known parts than to play forensic pathologist.