What to tell the exhaust shop.

Why not go 2.50 the whole way?
The exhaust gas is biggest right out of the header, that is where the bigger pipe is needed.
Thank you, I appreciate your input before I get this done. I was thinking that an early A body car like mine 64 Dart GT may not have enough clearance in the engine compartment for 2.5 inch pipes. My car is manual everything no power steering, or air conditioning. Has anyone else installed 2.5 inch pipe in the engine bay in a car similar to mine?