I was talking with a GM tech who has fooled with a lot of these older trucks and he mentioned a few things to ask you.
Did you eliminate the cat. converters?
Does your check engine light come on when driving, go out or stay on all the time?
Does is come on when you start the truck?
If you put put the O2 sensor back in, you will have a add an H-pipe (connector between the 2 exhaust pipes) and tap the O2 sensor in there so that it gets a reading from both banks of the engine. If there are cats and no O2 sensors, they will probably get plugged up from the excess fuel going through them. In open loop the computer can not adjust the mixture and run as lean because it is not monitoring the O2 sensor reading.
Can you put the oem exhaust back on, maybe you would see better milage. Dual exhaust is not help on these computer controlled trucks for mileage.