Slant Six Won't Rev over 3500 RPM

What do you have for mufflers? It sounds like either the fuel flow is restricted or the exhaust is restricted. Glass packs can be a problem in this regard it they deteriorate.

Is the throttle opening fully from the pedal? Sounds too simple but.....

Any chance that the cam was installed with the timing waay too advanced? It would be easy to check by looking at the valve positions versus crank timing.

It will tell you something on AFR at WOT to:
- Warm it up to full temp
- Swap in a set of new plugs
- Drive it at the 3500 RPM limit for 10-15 seconds
- Cut ignition and stop and look at the plugs
(The old-fashioned way AFR was tuned in the old days prior to O2 sensors....)