LS3 w/headers, which side do i put the O2 sensor bung in ?
it goes in the collector i assume or not ?

 

It doesnt matter but the drivers side is closer and less hassle to get to on most vehicles.

It has to go before the cat converter and with headers it cannot have a leak before it or it will skew readings. If the headers have two bungs per side it can go in the second spot if no cat converter is present.