Full throttle performance will prefer the larger exhaust but it’s hard to second guess what is going to happen with the cruising performance.  I’ll suggest going with the 2½“ exhaust system and leaving the option to restrict it at the back so you could run it with a variety of smaller openings.

Lorena, Texas (South of Waco)

 Have a mild modified 272(mild cam,headers.BT manifold with a 525 Barry Grant carb.) would like to install a set of Doug's electric cutouts but only come in 2 1/2" size or larger.Would a 2 1/2" exhaust system (with Flowmasters-40 series)exiting out to rear bumper be to large for the motor? KID