Okay, I'll weigh in here just one last time.
Pech, I suppose it's possible to replace the pan with the car on the frame. It is impossible to do it correctly and / or nicely.
Heck, I'd seen new pans welded on TOP of the old (rusted) pans. So, I suppose it comes down to what you desire as a finished product.
Coopman had mentioned about having the top in place. That's very good advice. I've restored enough convertibles to know just how important it is to fit as many pieces as possible prior to any welding.
The photos that Coopman had posted are of the most recent convertible I'd restored. To get to the point that is depicted in those photos, doors, hood, trunk lid and top bows were fitted perfectly.
The cloth top can easily pull the cowl assembly back 1/4" to 3/4" depending on the expertise of the installer. I plan for a 1/4".
From experience, I know approximately how much pull will be placed upon the A pillars and the subsequent door gap changes that will surely occur at the B pillars.
I'm not certain whether or not it shows clearly in the photos from the prior post, but those braces are adjustable. They are NOT "one size fits all" '40 convertible braces.
Carpenter and Drake sell the same pans. I prefer to avoid Drake any time I am able. The pans fit well and require only a small amount of modification. If this is your first "rodeo" I'd strongly suggest you buy the pan with the torque tube hump (tunnel) integral.
By the way, if I recall correctly, this is the same car that sports the rotten frame? If yes, then some comments and questions come to mind.
To boil those down to perhaps one... why would you bother replacing a floor pan on this particular car? It needs a frame, period.
I might hazard a suggestion at this point... sell this car and buy a better example.