What to tell the exhaust shop.

One last thing (maybe): I don't know the capabilities of your muffler shop, but many, probably most, will not be able to bend a 2.5 inch tailpipe without kinking it. Since the exhaust has cooled considerably by the time it gets back to the tailpipe, a 2.25 inch pipe would add little, if any, restriction to the rest of the system. Alternatively, although I don't see it specifically listed, I'm pretty sure you could order a single 2.5 inch mandrel-bent tailpipe from TTI, since they offer a complete 2.5 inch slant six single exhaust system.