For me I am very very cautious. So the answer from me is to leave the tub on the frame and change the floor pans. With this said I will now say that you need to release the tub from the frame to get the floor pans out and the new ones in. But this can be done one side at a time. And you only need to raise the tub a few inches on each side to do this.
I just changed a floor pan and sill in a TR3. Ten years ago I changed the other floor pan. Then I did it without releasing the tub from the frame. It was really hard. I would recommend that you brace the door openings or if you have to go deeper and replace the sills, as I did, and create a full square box of 1" square tubular steel inside the car before starting. If you are not removing the inner and outer sills, the tub should be stable without much support. So you would only loosen the bolts on one side and remove them on the side you are working on. Then replace them on the finished side but leave them loose and remove the bolts from the second side.
Maybe somebody out there can tell us how much trouble or trouble free it was to remount a body tub back on the frame and then get the body panels to match up. I tried to avoid this situation if at all possible.
Good luck.
Charley Fitch
