I wouldn't call the installation "unsafe" just yet. It apparently served the original owner just fine. Very few would go to the trouble of using full penetration welds for attachments like this. A one-side fillet weld can be plenty strong enough. As far as weakening the frame by grinding it off? No problem. If the frame was weakened it happened when the weld was made. Grinding it off will have no affect on the strength remaining.
I agree with the others. Just have your welder do what's necessary to get the new brackets where they need to be.