Cutting up an early VW convertible, unless it was so far gone no self-respecting Vintage VW Nazi would even look at it, would be kinda like chopping the top on a Duesenberg... They are just too valuable anymore to cut them up....
The support rails on a VW convertible are in the body, not the floor pan... the pans are identical between convertible and Sedan.
The body of s Sedan can be strengthened fairly easily with a chunk of 2x3 or 2x4 rectangular tubing, which is pretty much, the same thing VW used on their Beetle convertibles.