Exhaust on 64 Barracuda?

So finally getting the car to the muffler shop next week for exhaust work. It has a 318 in it and I believe factory manifolds. The guy asked me if I knew if the manifolds would accept 2.5 pipe cause that is what I was going to go with. Any buddy know if the will? I don't think I need 2.5 inch right now but the car is set for my dad but will want to do it with a built motor in a few years so I am trying to think ahead. Am I thinking this through correctly? Any suggestions appreciated.