Drivers Side Floor Pan Replacement

You will need to brace the livin' bejeezus out of it- with no solid top to it, there is NO structure holding it together other than the rocker panels and a rusted out floor.
Since it's already rotted, first thing I would do is get it up on jackstands (or hoist, if available). Measure and remeasure all your frame reference points to make sure it's square and true- adjust and shim your support points to get it in spec. Most converts at this stage of their life aren't true anymore even without rust damage. Make darn sure the frame rails are solid and not rotted ANYWHERE at this point. Weld in some X braces in the door openings. An X brace in the passenger compartment would be nice, but will severely limit your access putting in a complete floor pan, which it sounds like you're doing. If you're doing partial pans, access will be a bit easier.
Some may disagree with me, but with a convertible I would install your frame connectors now, while the frame is true- if your plans include them. Anything you can do to stiffen up the unibody structure before you start cutting will save you grief in the long run. The last thing you want to end up with is a swayback convertible when you're done.
And buy a mittful of good spot weld cutters.
Good luck with the project!