The 7029263 carb is not a direct replacement for the factory 455 HO carb. I would correct the lean issue with the HO carb and put it back in place. It is a special carb made just for those engines. Once the lean part throttle condition is corrected nothing out there will work better on that engine.
Also make sure that you plugged the hole in the air cleaner base after the carb swap. The large front vent tube on the HO carb sealed off any unfiltered air that would enter the engine as that hole is after the air filter.
I've seen a boatload of Pontiac engines ruined over the years and the 1971 thru 1979 Q-jets (and 1970 CA emission engines) used a large front vent tube that protrudes up thru the air cleaner base.......
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73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile),
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