Replaced engine poor fuel economy after

[youngbeamertech]

I have a 2015 Chevy tahoe 5.3 L83. I replaced the engine with a stock engine. Noticed poor fuel economy. Recently added Lt headers due to thinking CATs were the issue since they had 200k. I have disabled DoD and adjusted torque converter lockup 1-4 prior. Today I did the majority of the suggested mods for LT headers. But today I noticed my LT fuel trim are negative which might be the cause well before the headers. Any help would be greatly appreciated. Not sure what changed when I swapped out the engine. Otherwise the truck runs good, had no check engine light just mileage sucked compared to before. Installed 4 need oxygen sensors prior to LT headers. Thanks in advance!