TTI makes a full 2.5 h and x pipe system for 63-66 a bodys from the manifolds to the rear bumper [with notes for trans. xmbr mod,e brake hangar removal/mod,springs moved in notes] ,so i would say yes they will accept 2.5 flanges. Shane i don't know what to say sounds like you got hosed by a lazy exhaust guy. The p.o. of my cuda had Midas do the dual exhaust they f--ked it all up [2 inch pipe single to dual 2 inch with 2 1/4 flowmasters with 2 inch tails] im gonna have to redo it front to back with 2 1/2, it's a complete hack job.