Rust? What is rust??
(Ha, ha, greetings from the salt-free West Coast.....
)
Measuring at a wrecking yard is not a bad idea.
Are you working on a sedan or a wagon? I have a 740 wagon parts car that I will be pulling the motor from, and then scrapping, within the next month. If the section of floorpan you need to replace is not large, I could almost even cut it out of the bottom of my non-rusty wagon and send it to you. I do not know whether it is usually considered easier to weld in a section of used body or to just start over with fresh sheetmetal instead... but if you would like, as long as it is not huge and I can find the appropriate tool, whatever pieces you may need are yours for the cost of shipping. Hate to junk something that someone might be able to use.