As you allready know...a 318 is going to use a Y pipe right off of the manifolds. If you are "IN" with the guys at the parts store, have them order a Y pipe. You can install it and take all your measurements off of it and then return it.

Its been a few years since I've been a mechanic but I'm sure you can still get a Y at the parts store.

I used to work at an "Exhaust Pros" shop and my boss did a lot of custom exhaust work. Are you sure that you can run 2.5" right off the manifold? You may have to run smaller pipe for a short distance and then expand it. I think stock pipe was only 2". You should be able to go 2.25" off of the manifolds and then go up to 2.5" after the first bend or two.

This is the only picture I have of a stock 318 Y pipe. It gives you a little look at the passengers side but nothing else.