I have a 3" sports cat that I'm hoping to put on my 95WRX and mate to the existing 2.5" fujitsubo stainless system. The downpipe has a flat flange, but the rest of the system has a tapered flange that accepts the "donut" on the existing (original) cat equipped downpipe.

My question is, what do I need to put between the two exhaust sections so that they mate together without a leak?

 

I had a reducer (3" stepdown to 2.5")custom made and fitted by quicksave tyres in st annes - cost £60. Pretty much any garage that does custom exhausts can make you something that will do the job.