[Allen]

I've parted out several 2 and 4 door Novas.... the wheel base and floors are the same.

Marlin is right on the other difference.... primarly being that the roofline is slightly taller and creates a different rake for the front and rear glass.

The differences that you see in the doors... the rear door nests up against the rear wheel house, so the two short doors fit where the one long and quarter sheetmetal are.

Other 4-door specific parts include...

A-pillars, B-pillar trim, front and rear windows, rear seat bottom, front bench seat, side door glass, vent window assemblies, and a few more little things.

Common parts between 2 and 4 door cars include the whole front end sheetmetal and trim, all dash parts, floor pan, trunk lid and rear stuff (bumper, lights, etc), drivetrain, driveshaft.