when i had both my 351 stang and my 351 pinto .. the stang was/is a 5 speed and the pinto was an automatic .. a brand new carb on the pinto would spit back every time i stabbed the throttle ..the mustang dint have that problem with an old carb .. as you i tried every tune imanigable to make the pop go away .. then i simply swapped carbs with each other and no more problem .. the mustang with the clutch i could rev past the pop and the pinto never popped again .. if you have access to another carb give it a try ...