You can use most of the rear quarter from a '68-'72 Nova (which are extremely popular to restore, plenty of parts availability) on a '73-'74 Nova just the same. The only difference is the shape of the rear seat side windows/C-pillar, but normally you wouldn't need to replace that much of the quarter anyway. 1968-74 are all one generation of Nova so most stuff will interchange (in fact, lots of folks have converted the front & rear clips of '73-'74 cars into the more popular '68-'72 variants - only thing you can't easily convert is the aforementioned C-pillar area).