Are the metering rods / power piston assembly adjusted properly in the carb? Float level set correctly? Have you had a vacuum gauge on the motor? Can you rig one in the cabin and watch it while you are driving and check the reading when it acts up? Noticed your pushrod question - Have the heads been milled - if so how much. I had a 340 that would backfire over 4500 rpm - was running non adjustable rockers, was getting into valve float due to milled heads and block - adjustable rockers fixed that. I'd pull the sending unit from the gas tank and check the sock/filter - see if it isn't plugged or collapsed.