I RECENTLY PUT TOGETHER THE 350 TO 383 UPGRADE WROTE IN THE CHEVY HIGH PERFORMANCE MAGAZINE LAST YEAR. DETAILS - 4 BOLT MAINS,POWERHOUSE STROKER KIT WITH RING UPGRADE, .030 OVERBORE, BALANCED, 500/510 CRANE HYDRULIC ROLLER CAM, PATRIOT HIGH FLOW ALUMINUM HEADS WITH SPRING UPGRADE, SCORPION 1.5 ROLLER ROCKERS, EDELBROCK RPM AIR GAP INTAKE, EDELBROCK 650 AVS CARB., EDELBROCK MECHANICAL FUEL PUMP, MSD ELECTRONIC DISTRIBUTOR, 2 1/2" FLOWMASTER EXHAUSTS.
PROBLEM - WHEN I EXCELERATE GRADUALLY, THE CAR PICKS UP FINE AND HAS GOOD POWER.  BUT AT ANY SPEED WHEN I GET INTO THE THROTTLE HARD THERE IS A BIG HESITATION FOR ENGINE PICKUP. THE CAR WILL ALMOST DIE, THEN GOES REAL STRONG. I'VE TAKEN THE CAR TO A COUPLE OF GOOD MECHANICS.  ONE THINKS THAT THE CARB IS DUMPING MORE AIR THEN GAS WHEN THE SECONDARY CUTS IN, THEN THE FUEL CATCHES UP.  BUT THAT'S JUST A GUESS.  ANY IDEAS ON CORRECTING THE PROBLEM. OTHER THEN THIS BAD HESITATION THE CAR RUNS GREAT.  

check at the timing, or play with the distributor  

While it could be a timing issue, it sounds more like the carb isn't tuned correctly for WOT on your motor. Either the secondaries aren't kicking in right or it just doesn't have enough fuel at WOT.  

Check your fuel pressure at WOT it sounds like it's starving, or maybe flooding.  If you have good fuel pressure it could be the accelerator pump is not providing enough fuel or your bowls are running dry.  It may be also be a timing issue, any pinging (too much) or since it's flat maybe too little.

Good luck