What to tell the exhaust shop.

Ask the pipe shop, they may not have all the dies to properly bend 2.5 pipe. 2.25 will be easier to bend and tuck up out of the way. It's only 225 cubic inches, so 2.25 should be plenty. I would take a smooth bend, tucked 2.25 over a crimped up 2.5 that hangs low and rubs.
jus sayin....