When I did my exhaust, I used some of the 2.5" slip fit adapters from the auto parts store...but I cut them into 1" collars...no slag in the pipe...no reduction of inside diameter...easy to mock up...just tack one side...then slip the other pipe in the collar...get it the way you want it...then tack it...then pull the whole assembly down and weld it all...

______________________________________
'73 Montego MX 357C Homebuilt EFI, 9.5:1 comp, mild cam, 30lb injectors, A9L computer, Moates.net chip, 70mm TB, 76mm C&L mass air.