I welded some flanges on mine after reading about some fires/ extreme heat in the side sills while using clamps around cats (not necessarily Roes). The oval cats I had had expanded ends to fit 3 inch pipe, but the expanded portions were way too big for routine muffler clamps or band clamps to make a good seal.
Flanges will allow you to swap out cats for straight pipe if the need arises.
Flanges will allow you to swap out cats for straight pipe if the need arises.