The last Pontiac Firebird rolled off assembly lines in 2002, and that model shared virtually nothing in common with the 1979 Firebird.
If you take this car to a body shop, they should know how to track down interior components. You may want to find a shop that specializes in classic cars, and be sure to ask them specifically where they’re going to source their parts from.
Interior upholstery kits typically cost between $1,000 and $2,500, while other components will be priced all over the place. A steering wheel, for instance, will cost $150 to $300. This can be a rather expensive endeavor, and it could take months to complete, too.
An option we’d suggest is to hold off on the interior repairs until your dad knows what he’s getting, and then ask him how he’d like the interior done. Does he want it restored to spec? Or maybe throw in some modern flair? Maybe you can sneakily ask him without really asking him!