stock ecu and 440's...that's your problem right there. you're overfueling like crazy. 440's are almost twice the size of stock. the reason it's showing lean is because there is a sh#t ton of unburnt fuel and no o2 in the exhaust since a large volume of it is gas.

burning excess fuel=rich
burning too little fuel=lean
not burning fuel=lean becuase it reduces the volume of exhaust gas/air in the pipe.
black on bumper and backfire between shifts=too much fuel and fuel burning in the exhuast pipe.

need to socket/chip the ecu and tune for those injectors or swap to stock 240cc injectors.