They should be clamped on. Just get 2.5" pipe and replace the cats with that and keep them in a box for emissions days or if you get a fix it ticket from a cop in a state that does not have emissions. Just FYI it is a federal offense to remove them if a cop REALLY wanted to be an *** about it.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.