The Performer RPM isn't helping. A regular Performer or 68-70 cast iron would be better suited to bottom end. Have you tried any more initial timing? 12-14 degrees? Get a 3" air filter. That cam in a 455 shouldn't need much stall.
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frittering and wasting the hours in an off hand way....
1969 GTO, 455ci, 230/236 Pontiac Dude's "Butcher Special" Comp hyd roller cam with Crower HIPPO solid roller lifters, Q-jet, Edelbrock P4B-QJ, Doug's headers, ported 6X-8 (97cc) heads, TKO600, 3.73 geared Eaton Tru-Trac 8.5", hydroboost, rear disc brakes......and my greatest mechanical feat....a new heater core. 
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