2.5" exhaust will maintain flow velocity & will perform better in low-end & midrange, plus with the right mufflers, will perform just fine in the high RPM's as well. If you want to use Ultra Flos, use them in conjunction with some 16"-18" resonators in the rear.

On the TTI note, I have a buddy with a 70 Challenger 440-6 & the whole system headers back. One of the primaries makes constant contact with the torsion bar & the only way to remedy it is to dent the primary. Also, he got the 3" system with Super Turbos & it sounded great at first, but once the packing blew-out of them, they got SUPER loud. My chambered exhaust on his car even in 3" would be quieter than the Super Turbos now. Plenty of room for resonators in the tail pipes on an E-Body.