95 chevy 1500 350 hard running after new exhaust
 

I put a new exhaust on a 95 1500 chevy with a 350. The exhaust is a dyno max system brand new. There was a duel exhast on it which was rotted out and was broken completly off at the manifold. Well i put the new system on and now its not running right. People are saying its because it was a duel and now has a single exhaust on it. I did not put a new o2 sensor in it and thats what i think it is. I plugged the o2 hole rate next to the cat and it has the old sensor on the manifold. They say that its not getting enough oxygen now cause its not a dual exhaust makes no sense to me i think it has too much fuel now cause it was running wide open and now its loading up being closed up. It is back firing and was actually shooting flames out after we took the muffler of and ran it a while. Any ideas let me know