Rich LS1

I have a 1964 vette with an LS1 and 5 speed. The engine runs good but I am fouling plugs very quickly. It is a 383 with 78mm FAST TB and manifold, .572 Comp cam, some head work, and custom sidemount Hooker headers. I have 36lb FAST injectors in it. The local shop has hooked it up and all the sensors appear to be working OK, could the flow thru mufflers in the sidepipes not allow the O2s to work well, they are mounted in the side of the collector in front of the pipes. I also have some 28lb injectors would swapping help? I am open to suggestions.
Steve