Truck stumbles and almost dies after carb rebuild

[Arl37]

69 Chevy 307. It had a two barrel and ran fine. In a fit of being adventurous I bought a performer manifold and rebuilt a old Holley 600 CFM barrel. vacuum secondary.
Truck had new plug wires and HEI before the 4 barrel installation. Runs smooth at idle but when you try to accelerate, even mildly, it backfires and almost dies. When it does pick back runs fine. Cant imagine it is timing. Checked timing and it is 4 BTDC.
Must be carb. Where do I start looking?
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