Performance loss from headers to HP manifolds

I arranged to have the weekend off so I just may go to the swap meet on the 14th. Last time I went, there wasn't a whole lot of Mopar goodies around. Mostly Chevy and Ford. O.K., here is my set-up:

340 bored .030 using stock cast 10.3:1 pistons , block decked .010
J heads with a mild port and 2.02 intakes installed, milled .035
Edelbrock LD340 intake with Holley 3310 750 cfm vacuum sec. carb
Comp cams XE268 Extreme energy cam
Felpro .050 head gasket ( needed more thickness so the valves wouldn't hit, being cautious!)
Final comp ratio 9.8:1 The stock pistons are not near 10.3:1 as advertised.
Mopar performance headers
Sniper NOS 125 HP jets.
2/14" exhaust with H-pipe into Dynomax Super turbo mufflers and tailpipes.
3.21 rear gears
727 trans with TF2 shift kit and 2500 stall convertor.

Maybe I need the headers with this set-up, maybe the cast manifolds will choke it too much. More input needed please.