What you just described where you unbolt your cat and in place put a test pipe and vice versa when smog is up is exactly what im looking for. Is there such a kit or is this sort of thing custom?

In your opinion would it be cheaper to buy a cheap 4" turbo back and have an exhaust shop install some flanges where the cat would sit or just buy a down pipe and have an exhaust shop do the rest?

I have no idea if there is such a kit, we did ours the good old fashioned way, a little imagination and a welder. To do it the way I did is about $250 (welding the flanges up yourself) including the $100 for the downpipe, straight pipe bent by the shop, 4 of these Exhaust Flange, 4" Tubing 3 Hole - DPF and EGR Kits and the necessary hardware which can be picked up at your local hardware store, nylocks and lock washers have been holding on mine for a good amount of time.

I'm pretty sure there are kits available with the option to use your stock cat. MBRP comes to mind.

MBRP S6206AL MBRP Installer Series Aluminized Exhaust Systems For Diesels

Stock cat retained, less than $400, before shipping anyway. I had a straight through, 4 inch Dynomax tube muffler as shown in the ad, when I took it off there was no change in how loud the truck was.

Mdub I think he was asking if there was a flange kit to have a bolt in/out cat

Agreed, I guess I didn't explain that very well, but the stock cat is bolt-in/bolt-out, and when you buy an aftermarket exhaust designed to use the stock cat, typically you can bolt it in or out, and there is a "test piece" for offroad use only that you can swap in, in it's place, so you have the option to go either way. Any system that uses the stock cat will be bolt-in/bolt-out.

Has anyone passed the smog test with a straight pipe my truck doesn't have a Cat and am trying to figure out if I can straight pipe my F-250 and still pass smog. Thanks.