As Tom said, the Custom was basically a fancier trim level than the regular Nova.
The Custom package was available on all body styles (hatchback, 2-door, and 4-door). Scott's NovaResource site has a list of all the items that were part of the Nova Custom package --> http://www.novaresource.org/custom.htm
My Custom 73 Hatchback came with the rubber impact strip on the bumpers, the bright rocker panel moldings and the chrome with black insert body moldings.
Only real obvious trim differences I can think of.
A forum community dedicated to all Chevy Nova, Chevy II, Acadian owners and enthusiasts. Come join the discussion about performance, modifications, classifieds, troubleshooting, maintenance, and more!