This is how I do them
When I put a cage in a G-Body, I box the frame rails where the cage attaches, use aluminum solid bushings, weld plates to the floorboard and weld the tubing to the plates in the floorboard and the frame. If you use rubber or polyurethane bushings, I'd suggest you leave 1/8" around the tubes where they pass through the floorboards and fill the gap with silicone. I make my own aluminum bushings by getting aluminum bar stock and drilling it, then cutting it into bushings the thickness I need. I'm sure not everyone is going to agree with the way I do it, but I build cars for Division 2 and they tech hard here in the southeastern doorslammer conference. My cars always certify, and they are solid.