2.25" or 2.5" exhaust?

I just got a set of older Doug's Headers and am trying to plan out the exhaust system for my 67 Dart. I'll be running dual exhaust of course but was wondering what pipe size that you would recommend for a street built 273. I was leaning towards 2.25" but was also considering 2.5". There is the possiblity that I could eventually put a hotter 318 in it (for no other reason than that I have a couple of 318 blocks sitting around itching to do something) but probably nothing bigger than the 273 or 318. The headers have 2.5" collectors.
Thanks
Kevin