With LT's, and say either a catted H or X.... you install NEW hi flow cats. You never re-locate the oem ones..which are useless anyway. OEM exhaust manifold on the 05-10 cars is just a scant 2" diam. That goes into the skinny oem cats at base of eng bay. Output of oem cats is 2.5". Then it's 2.5" all the way back.
Still baffles me why hi flow cats, or moving a hi flow cat rearwards has anything to do with reality. High flow cats are big diam things, like 4". They are shorter than oem skinny cats. Short + fat has as much ( and more) internal volume as long + skinny. The primary difference being, the high flow cat will flow better,simply cuz it has more parallel paths..and each shorter to boot. In the old 5.0 rag, they compared no cats to catted H / X.. and the difference across the entire rpm range was nil. 1-2 hp in favour of no cats at one rpm, then 1-2 hp in favour of hi flow cats at another rpm. California has gone beserko with its rules.
Daughter was at a Subaru dealer a few years ago, looking at new cars ( I was with her, told her don't buy a subie). The old geezer who owns the place, mentioned there was also an optional.... 'near zero california emissions device'.. which was a whopping $1100.00 option, wtf ? I never did find out if this 'device' replaced the oem cat..or was in addition to it. Of course with all the enviro geeks in this town, with their heads up their ass, it was apparently a hot seller. I suppose what's next in california will be mandated 100% electric cars. Then they get to fry the electrical grid, then build a 1/2 doz nuke plants to run the electric cars. Buddy of mine, his daughter bought an 100% electric Nissan Leaf. Her (every 2 months) electric bill will make u gag. It's 8.5 cents per kwh for the 1st XXX kwh, then the rates jump to 12.5 cents per kwh. I have never paid the higher rate in 20 years. The Nissan Leaf fans all get screwed..sucking 7.3 kw for hrs on end, typ 4-7 hrs per night.