rudes

it doesn't matter as long as its after cat, but within an inch is good.

when you replace all 4 with 2 high flows, ull probably have to buy an 02 extension to make it fit behind each new cat

there are 2 sensors that measure the air:fuel ratio, 1 on each side in front of the Pre cats. dont ever touch those. its probably different on ur truck than it is mine due to the obs and nbs, but are your sure that the 2nd set of sensors are in front of the rear cats? there arent anymore? my truck has them in the actually rear cat themselves. i would check it one more time, ive never heard of sensors being before the rear cats. when you do install them, if the cats you have dont already have the 02 budges in the cat themselves, weld the sensors after the pre cats and you wont get a CEL. worse comes to worse, just have any mechanic shop clear the code, but i doubt it wil come down to that.