Have you considered modifying the rod linkage? It seems to me length of the carb rod and the lever Throttle arm lever can be changed for better travel and geometry.
I read that some Firebird carb linkages were intentionally made so they would not open all the way. GM was afraid the Firebird would infringe on their darling GTO. They had the same exact engines, but were restricted from making the same power. What I don't know is exactly how they did it. Your problem may be that restriction.
