I can't figure it out, maybe you can?

Broke two rocker arms on my 1973 Dart Sport 340 bracket car.
Pulled the heads and replaced the valve springs, hyd lifters, rocker arms.
Went out and it wouldn't rev past 5200 rpm.
I replaced the orange spark box with a chrome one and went out last night.
Better, but still won't rev past 5500 rpm.

It pulled the wheels on the launch, but just won't rev past 5500 rpm.
This car ran 12:60's before breaking the rockers.
Since then it hasn't run faster then 14:50

I have a thermo Quad on it that's worked really well, but I'm thinking something may have gone wrong with it, making the motor lean out and causing the lack of rpm?

I've checked everything else. When I had the heads off, everything looked good. I've checked the timing and it's dead on. The car starts right up and idles fine.

Any idea's on what the problem might be?

Thanks!