Thanks for these additional perspectives. I've rebuilt my Rochester 4BBL and eliminated the restrictor flange on the secondaries and rebuilt my stock throttle linkage to remove the play, so I can eliminate these two factors.

Remaining factors are: a. the plastic bushing on the firewall bracket is worn and has flex, and b. the accelerator pedal which I cannot adjust (may be welded in place!). I'll replace both the firewall bushing and the accelerator assembly to try and get to closure.

FWIW I have seem a few aluminum accelerator assemblies that are more adjustable but $20 is a lot for a gas pedal and I do not want bling under my feet for a weekend driver. I will try the $60 assembly on Classic or Firebird Central (cannot find better quality).