Holley 750 backfire.

I did a 440 w/285* solid and tried .509 before that, same intake as yours and 3200 stall w/4.30 gears....it liked the 750 with 28 shooters and red cams, 71 primary and 82 secondary...PV should be half or just above half of the manifold vac reading in gear foot on brake.

as for the staggard jetting, maybe he though that was the fix for a bad intake it may have once been used on.lol...or maybe he flowed the carb and found it to be out of spec...who knows.