I bought a new Magnaflow catalytic converter for my 1979 Alfetta Sprint Veloce. These converters don’t have the flanges like the OEM converters because they are meant to be welded in.
They’re also a lot cheaper (~US$65 vs $200-$250).
I cut the flanges off the old converter and ground them down so they’d fit on the new converter. Robert at Bay Muffler Service welded them on for me.