After buttoning the fuel system up yesterday, setting fuel pressure, and some tuning completed, there is now more work to be done in the engine diagnosis.
The fuel system needed an upgrade, and received a Demon 750 carb. Then the entire fuel system ahead of the carburetor was upgraded to compensate for the small factory lines and weak MP mechanical pump.
The ignition was weak as a points system, and was converted to MP electronic. This was an upgrade but not quite good enough, and was upgraded further to the MSD system with the Taylor 10.4 wires.
At this point the engine idles strong with about 11" at idle. The throttle snap is tight and crisp, even with the stripped accelerator squirter. Though there is a nagging problem at around 3,000 +/- rpm, the engine shakes and doesn't seem to want to rev much higher. Below this point from idle the engine is very strong, and the engine shake is non-existent (aside from normal cam induced shake at idle).
I assume that now I'll need to go through the valvetrain to inspect for weak springs, collapsed lifters, or worn cam lobes. I may just swap in a new set of the correct springs and see if that helps.
The work completed thus far in diagnosis has me somewhat baffled as the vacuum gauge doesn't flutter and doesn't show a weak signal - which is common with valvetrain failure. The exhaust temperature at the headers are all fairly consistent as well between all cylinders. The only thing I have yet to verify is voltage at the coil and MSD power feed at the rpm where the shake/stumble occurs.
I could also try a different rev limiter in the MSD box, in the event the one installed is faulty?