To replace the floor pans in a convertible, you will have to measure across the door opening diagonally (top of rear quarter to bottom of cowl, top of cowl to bottom of rear quarter). Then measure straight across the top and straight across the bottom.
You want to let the car sag just a little so that the top of the door opening is about 3/8" larger than the bottom. When you weld the pans in with the car in this attitude, the door seam will show the difference with a larger gap at the top than the bottom. After about 2 months of driving and sitting back on its wheels, the car will straighten out, the pans will seat in and the door crease will move to a uniform gap. If you don't do this, the same settling will occur and the seam that was straight when you first mounted the door back on will now be too narrow at the top and the door might start hitting the rear quarter at the top.
Remember, the floor is the primary structural component in a convertible and when you remove the pans, there is little support left for the body. You can minimize the strain on the body by replacing only one section of floor pan at a time, but you still need to measure carefully to make sure that there is no body flexing.
If you've never done this before, you may not want to start on a convertible. It may be worth your while to farm that part out to an experienced restorer and then finish the rest yourself.
As for the price of the car, $1400 seems like a good price. There may be better cars out there but it sounds like you know exactly what this one is going to need. If you feel comfortable with handling the floor repair or have the means to pay someone else a reasonable price to do that part, I'd say go for it.
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