Love that Chieftain, it's a beauty! Nice with the 389. Bummer about the shop fire!!!
Agree with Stuart about California Pontiac Restoration (CPR). I've bought a lot of parts from them. They know Pontiacs. The other Pontiac parts Supplier is Ames,
www.amesperf.com. You may run onto Kanter in your search for parts, I do NOT recommend them for suspension parts.
Regarding interchangeability of Chevy/Pontiac parts. Lots of body parts interchange, including glass, doors, seats, and most interior trim. All door parts should interchange including door panels and armrests (trim will be different). I believe sunvisors will interchange. Floors are not the same so carpeting will likely not interchange.
Virtually nothing else interchanges. Fisher Body made the body for Chevy and Pontiac. The body then went to the respective Chevy and Pontiac assembly plants where Chevy and Pontiac each went their own way with just about everything. Mechanically, Pontiac has much more in common with Oldsmobile.
You may want to get familiar with
www.PontiacSafari.com. Check out the "Links" page and the "Pontiac Garage" page. You'll find the shop manual, Fisher Body manuals (Body by Fisher Service News), Master Parts Catalog, and more.
Larger tires will probably not rub but you may find it difficult to install them on the rear.