Replacing headers with after maket parts

[handfrd123]

Hello fellow bikers, can someone point me in the right direction. My GTR has a rusted set of headers mainly blowing where the exhaust fits. So after searching e-bay dealers etc who had no stock, I decided to go for a new set from Black widow headers.
So after re-fitting the o2 sensors, replacing with new header gaskets, headers, and fitting an SP engineering carbon end can/link pipe (Baffle in) Then took out for a run, then noticed the exhaust backfiring on over-run a lot worse with the baffle out. After a bit of advice, I did the o2 sensor delete and removed the sensors and blanked off. After this did another run which did not make much difference. Now, where do I go from here, ECU flash? Power commander?
whats are the options? 

[Boomer]

To stop the backfire and popping when you throttle back, you need to block the hose that connects the airbox to the top of the rocker cover. It feeds fresh air into the headers to reduce emissions and with low restriction exhausts causes popping.