RRE downpipe

On the RRE website, it says that their downpipe will bolt up only to a few certain cat-back exhausts. Since I already have an evoIII o2 housing(not installed yet, just sitting in my room), I'm definitely going with the RRE downpipe(long) since they make them fit directly to the evoIII.

Anyways, my question is this: I will be getting the rest of the exhaust past the downpipe custom made, and I am curious if the RRE downpipe will bolt up with the custom exhaust. Also, why exactly will their downpipes only bolt up to a few brands? What are the fitment issues with the other brands that cause them not to bolt up to the RRE downpipe? If it's just the difference in the flanges, I am thinking that I could get the apexi N1 flanges and the custom exhaust should bolt right up, correct? Thanks for the help! :)

Buy a flange from RRE to have welded onto the exhaust your going to have made..

The reason is they use a FULL 3" flange, not some shyte stock sized flange that many aftermarket 3" catbacks use. See thier page on "fake 3" exhausts." ;)

Order the Apexi flange from them to start off your custom catback and it will work beautifully. They will be identical flanges, and they are 1/2" thick, so you cant ask for a better joint than that! :) They sell a nice gasket too. I actually got a copper apexi sized gasket from AGP though, and that thing is sweet at my o2-DP conection. I might get one for the DP-catback.

Edit> In case I wasnt clear, RRE sells all these parts on thier DIY pages. The flange, and gasket, etc.