When I did mine, I had sent the body off to be plastic bead blasted. While it was gone, I sandblasted, cleaned and painted the frame. By the time the body came back, I had the finished frame on wheels again and was ready to drop the body back down on the frame to replace both inner and outer rockers (using all new body mounting rubber pads).
After all the welding of the floor-pans and rockers, I pulled the body up off the frame again and rolled the frame out of the garage so I could paint the undercarriage of the body.
If you are using a home-made frame out of lumber, you would need to make sure all four corners of your frame are perfectly level and have intermediate supports with wood blocks and wedges under every floor mounting hole before beginning cutting out the old rockers. And it would need to be high enough off the floor to enable you to crawl under the car to cut and weld on the inner rockers.