What Bodean said.
I have a 14" x 6" hole where my rear floor meets the sill. It had a thin piece of plywood over it when I got it in 2003. The rust was treated with converter and painted to stop or slow the progress. The plywood was removed in mid 2010 when I built the storage drawers in the back. That big hole and a number of small ones along the edge where the fender meets the floor are all still there. I will replace the rear sill, floor, and patch the other holes some day, but I'm having too much fun driving it right now!
With a LOT of great guidance from CLC members, I went with this progression:
Safety & Performance: Get it stopping, steering, and running good
Maintenance: fluids, knuckles, etc
Enjoy driving it!
Upgrades based on your driving experience, intended use, and preferences
40s are fun to drive no matter how they look or even how they smell! Get it going, turning, and stopping and let the fun begin.