how come you have longer pushrods?  

.100 pushrods are required when using RPM heads with Stock Rocker arms.  I also have a set.  

I don't know if the 600 performer carb is the Edlebrock Q-jet or the Carter, but whichever, the carb is dropping lean on the secondaries. whether this is a secondary metering problem or a primary problem, I can't tell from here. But basically this needs to be thought of as a secondary problem first. As they are opened more air is admitted to the engine, if the secondary metering isn't keeping up to fuel requirements, the mixture drops to the lean side. This is accompanied by sharp sounding sputtering and back fires thru the carb. On both carbs you have to control the rate the secondary airflow valve opens, if it's too loose you can pull air faster than the fuel meter circuits can deliver fuel and get a momentary lean out. You can hide this with the accelerating pump, but it's better to slow the air valve down a bit. Also you don't want too much idle throttle opening, unless you've got the Carter with an idle air bypass valve. You've got to keep the bottom of the butterflies just at the transition ports. This also applies to Holleys.

I'm speaking generically between the Q-jet and the Carter. For a Q-jet, this can be an alignment problem with the secondary metering rods not coming up fast enough/far enough, or they're too big which makes the jet opening too small for the air flow. If an AFB design, the jets could be too small or the boost venturi is loose and leaking air into the secondary metering circuit.

On the primary side of either carb, those metrering rods might be out of adjustment as well, by the time you're hitting the secondaries, the primary metering rods should be commited to the "power" enrichment setting which is their smallest diameter. If they're not all the way on this step the total mixture will fall lean unless the secondaries are running really rich. On the primary side this can lead you to a vacuum leak to the control piston(s) or excessivly stiff metering piston(s) return spring that dissrupts its function. Talk to Edlebrock's tech help.

Then of course then can be dirt of gunk caught in a circuit, or the fuel pressure doesn't keep up with demand, usually because of a bad pump or plugged filter. Both the Carter and the Q-jet have a filter in the inlet that is legion for plugging up, so check it out, it's behind the inlet nut. Float levels can also be a problem, a little low might not show on the primaries but when the bowl has to feed both P and S it might not be enough. Both carbs suffer from primary lean out on hard acceleration as the G forces push fuel to the back side of the float bowl(s). The Q-jet has a splash shield, it has to be there or this is a big problem.

There's more esoteric things that go wrong but these above are the most common ones. 

Holley as a cure all, where super good gas mileage isn't the prime concern, the Holley is a lot easier to debug than anything else.

Bogie