Love the car i had almost the same setup. 71 gp sj 455 ho 472. Edelbrock heads built block. Same issue with old exhuast car was flat. Installed flowmaster dual kit with 40 series. It made as much power as open headers. Think i got a kit for a g body and some extender pipes and it was bolt on. Mufflers in front of the rear end.
Glass packs always kill power. They have onthe to cars i tried them on.