Been scratching my head and need some advice

[mr402]

The basic problem is when under a medium load my truck will pop through the carb. It rarely happens in 1st gear. Never seems to happen above 2500 RPM. The pop happens during normal acceleration or when going up hill at lower speeds (under 2500 RPM). I can accelerate lightly and it usually won't pop. If I hit the gas hard it won't pop either. It idles fine and will cruise very nicely at 65 (2500) RPM.

The carb was an Edelbrock so I tried replacing springs etc. but finally gave up and put my old Holley back on (6210 spread bore and installed a Holley renew kit) . No change. As an aside note it had been running with the Edlebrock for a few years.

It does have an adjustable vacuum advance so I put it a little more advance which seemed to make it worse. Then tried disconnecting the vacuum advance altogether. Nope, still popped. While still chasing the advance theory I put the stock weights and springs back in the HEI distributor. No Change.

I've used carb spray to check for vacuum leaks. Everything seems to come up OK.

A few other basics.
The engine is a 402 that was rebuilt about 80,000 miles ago.
Modifications include an Edelbrock performer intake, HEI ignition, the previously mentioned carbs, and advance mechanism changes (which are now back to stock). The exhaust is basically stock. All modifications were made some years back and it ran OK. The cam was basically referred to as an RV2. Overall pretty mild and kept power in the lower ranges for towing etc.

So after his long winded explanation what do I check next? I'm pretty much at a loss as to where to go from here.

To make matters worse I'm moving and really need to get my GMC running right so I can tow my other projects to their new home.

Your advice is appreciated.