Mandrel bent 2-1/2" would probbly be the best option, and it would probbably flow as good or better than crimp bent 3". The 2-1/2" is easier to fit too.
If you had 3" mufflers then I would say use 3" tube.

Just for comparison info, my Charger 451 stroker originally used 3.5" crimp bent tubing with Dynomax race bullet mufflers that dumped under the car before the rear axle. The car was just way too loud for the street, so I installed the TTI 3" mandrel bent exhaust system (used my old Hooker headers) with the Dynomax SuperTurbo mufflers, and tail pipes over the axle and out the back like stock. The quieter and smaller diameter TTI system did not make a difference in the cars performance at all.