[speed220mph]

HSS427: Air leak, as you've already addressed, is the first possibility. As for a leaky intake, if the manifold has a common plenum chamber, backfire should occur on both sides. Another scenario, and sadly one I encoutered, was misfire in a cylinder, causing excess fuel to enter the exhaust, which exploded when it joined with hot gasses from the other primaries. Problem was coolant leaking into cylinder on decel, cooling or leaning the charge and causing the misfire. A leaky head gasket could also cause the problem.

I suggest you pull the plugs on the passenger-side bank and check for differences. Rather than a compression test, do a leak-down test. Have the radiator cap off so you can see if there are air bubbles in the coolant when pressure is applied. If not, the problem could still be a leak, but air that leans the mixture, causing a missfire on decel in one cylinder, so listen for a hissing sound in the crankcase and carburetor. Good luck.