High RPM miss

I'll try to keep this as short as possible. 74 Dart bracket only car. 440/727 I am having popping through the exhaust at about 3,000 rpm that turns into severe backfire at 4,000. I have replaced all of the factory wiring except for the lights. I have a factory electronic dist., MSD model 6200 box, Autolite 86 plugs gapped at .035", GM one wire alt.
Things I have tried to fix the miss-fire:
New plugs and wires
New cap and rotor
Replaced dist. with a working one from my street car(Pertronix conversion)
Replace MSD coil with Pertronix from street car
Bypassed MSD box and only used Pertronix dist. and coil
Replaced Fram type fuel filter(old one looked clean)
Checked screen on inlet side of Holley Blue pump
Fuel pressure is at 7 psi at idle drops to about 6.5 with rpm increase
Replaced intermediate shaft bushing when I noticed the timing was bouncing about 2*
at idle and about 5* at 3,000 rpm. Initial is about 18* total bounces between 35* and 40*
at 3,000 rpm. None of the above changes have had any effect on this condition. Any help is greatly aprecciated.