i have an 85 that i dropped a 2000 4.3 and transmission in. i stuck the 85 t case on it but now i cant find the last o 2 censer plug. i had to remove whole harness because it was for the t case. if the plug was on that harness, is there away i can just not have it plugged in and work or will i lose mileage and stuff like that??

 

Considering the years difference, your 85 prolly had the 2 front before cat o2 sensors, the last plug that your looking for is prolly the 1 for after the cat, if thats the case then you should not need it. Someone can correct me if I am wrong