Re 73 G/A:
1: 73-77 fenders same, trunk lid is 73 only(G/A & Tempest)
2: Sport mirrors are A-body specific.
3: Yes for dash & seats and some trim but not door panels and power switches.
4: Any Pontiac vendor with GP catalog will have some interior pcs. Most GTO/Tempest catalogs stop at 72. Motorealm has some limited stuff available.
Try the "A-Body Resto Forum & Classifieds" @ users.boardnation.com. Not alot of traffic there but could be helpful. Their forum is 73-77 A-Body specific. Also try
www.TachRev.com.