I just gutted my 74 sovereign
...I cut three rows down the subfloor, middle and 4 inches from edge. I recut the edge closer to c channel with sawzall right near it...was able to pry old plywood in chunks out with a nail puller, flat bar and hammer. This was time consuming but worked. Bolts and screws have to be cut out with zip disk or sawzall, careful with sawzall if it bounces you get a nice gouge in lower belly pane. did that once oops... Have it set on off cuts of plywood on all outriggers to keep hull stable. Adding new steel in center of frame for new subfloor. ALWAYS have edges of subfloor on frame, glue together at all seems. Always have new subfloor sideways, never put plywood the lengthways, it's shear strength is from end to end. So as the structure moves it can absorb the vibration. I will be giving my new subfloor 3 coats of protection, edges is a must. Use cardboard templates to get curves right and cut plywood with a jigsaw to match. New bolts through c-channel through subfloor on edges is a must. Once floor is in you can safely remove belly pan edge pieces and bolt floor down, then just pop the pan back up. I will also be using pl 300 subfloor glue where it contacts frame for massive extra strength, along with new screws.