Well, I'd buy the cat first and see. Is the RT cat bolt-on? RT is really expensive. For $100 less you could go with a Carsound Hi-Flow Stainless steel cat. If you want cheaper then Summit or JEG's sells performance cats in the $50-70 range. I'm not sure if they're stainless steel though.
If no one sells a bolt-on(clamp on in your case.) You might have to get a universal model with a long input pipe, Then have it flared to slip over the downpipe to the Header. The down pipe should come with the Header. You might want to email Banks and see what they say and what they recommend since you'll be using all new parts.
I have already purchased the RT cat. It is a clamp on. Although, I am pretty sure I can weld it. Banks does not make a head pipe for the 97 TJ 2.5L as the head pipe is stainless steel from the factory. The issue I have is there doesn't seem to be much room between the OEM cat and the head pipe hanger (see image). I was wondering how I am supposed to deal with it... Cut the head pipe upstream from the hanger or cut the head pipe at the cat and clamp it there (just doesn't seem to be much room). I would like to keep the hanger intact.
