How far is to far gone?

I have a 1973 Dart Swinger that i had since i was 16 (now 30). After about two years of having it i got hit in it. The wreck bent the frame on the driver side and buckled out the quarter and messed the rear valence. But were it has set for so long the floor pans are gone and needs trunk extentions and other small body work needs. But i'm just wondering How Far Gone Is To Far Gone? And i was wondering if any body on here has rebuilt a car that has been pretty rusty too?