QuickFuel 880 Surging/Missing At Wide Open Throttle

IMHO, the fact that the engine ran well with the Holley at the top end with WOT says that the ignition is adequate. It maybe could be better and fire a wider range of fuel mixtures, but it obviously can work.

The smaller jets make some sense with a higher altitude. The backfire upon starting and the barely running says that something else was wrong besides the jets. The backfiring from the exhaust is due to excess raw fuel in the exhaust; the jet change would not do that as the main jets aren't in the pix when just simply starting the car. So I'd go back to the leaner jets and figure out what was wrong: a leaky base gasket, or something else is going wrong when you re-assemble the carb and causing problems IMHO.