ERA undercar headers are a little unconventional but basically the collectors are 2-1/2 inch and I ran 2-1/2 inch all the way to the rear bumpers with non-crush bends. A 2-1/2 inch exhaust system with low restriction mufflers will flow a lot of exhaust and support a lot of HP.
I would be leery of trying to run anything bigger than 2-1/2 inch as you are also compromising ground clearance. I used 2-1/2 inch deep mufflers by Spintech to maintain as much clearance as possible. If I were re-doing it I would have gone to 2-1/4 inch tail pipes from the mufflers back. By that point the exhaust temps have come down enough that a little smaller pipe won't compromise overall flow. It wouldn't have been so tight in the rear wheel wells either.
