408 fuel ?

fact is, you're running two carbs instead of one. fuel volume will be critical! usually you need volume to feed two carbs. a stock mech pump doesn't have enough volume to do the job. also larger dia line is usually required along with a good electric pump. I seen the inside of a stock mech pump off a 409 chev, it had two valves per side and they were bigger dia versus a regular mech pump.