do you think changing the manifold cats by cutting them out and welding in

The reason why you have a P0420 code is because of the catalytic convertor is inefficient. The rear O2 sensor reading should not mirror the upstream O2. It should flat line after it has warmed up. Unfortunately Wakler Exhaust does not have an aftermarket listing for the cats. There's a way to try to clean the cats through a special chemical. It requires a professional shop to clean them. Contact me at jmgrhm30@gmail.com.

Do you want to remove the cats because that is illegal.