sounds like you're pump shot is not working... With the engine idling, out of gear, rev it quickly... see if you get squirts of fuel into the barrels from the squirters. See if it's a consistant streem, or choppy, or no squirt at all.
... if it doesn't look clear, take a large philips screwdriver, undo the screw on the squirters, pull the squiters off (becarefull, they is a needle underneath that might get pulled out with it). Use some carb cleaner spray to clean the squirters, put them back. While there, take note of the number stamped on them... I think stock size is #31 squirters... you might need something bigger.
When the carb was rebuilt, would you have happen to know what kinda pump diaphrams they put in? You might need to go from the generic 30cc, to a larger 50cc pump diaphram, with the larger spring...
You should also see a Red or Orange pump cam on your throttle linkage assembly... You might need to use a larger Green or Blue one.
what size jets are in there?
is this a Vacuum secondaries carb? ... if so, your secondaries vacuum spring might be too stiff, not allowing your secondaries to open in time. I don't imagine it's the opposite situation, as a big block would not struggle if secondaries engage too quickly.