I am in a 73-77 GP group. You will find lots of helpful info there. Yes Nos parts are somewhat hard to find but the demand is getting better so there may be some mfgs reprducing some things. Performance years and ames both have catalogs you can order. Here is the link to register. We all buy sell and trade from each other. There is usually a parts car going around. They are nice cars.
http://autos.groups.yahoo.com/group/73GP77/
Good luck and if you have any ?? ask me.
Jerry