1) Disc brakes on front can be had from several donor vehicles. You will probably have to replace everything from the firewall onward. If you do decide to go for a copmplete kit, there are several good kits, and - unfortunately - one to avoid. (SSBC, I believe is the correct acronym.) Many of the guys here have done their own swap and can highlight the pitfalls. Rear discs are cool (talk to Brett "1stgenbirdman" about his) though not totally necessary.
2) Shoulder belts were an available option in 1969. You can get them from repro houses. There are no mods to the seat for this. You simply bolt a second buckle to the floor on the console side. I forget where the shoulder belt goes. (I have a convertible and I'm pretty limited in my knowledge of body, trim and interior details.)
3) PW were an option, also. Terry (69firebirdman) probably has a set - ask him. PDL, I am not so sure about.
4) 350 upgrades have been talked about extensively this week. (Must be your lucky day.) Check some of the other posts in this section.
5) If you want original parts, 69Firebirdman and FirebirdBill will probably be able to come up with everything you need. Otherweise, go with 230Sprint6's suggestion of picking up Ames, Classic Industries, NPD and PY and catalogs. (Sorry if I left out anyone else's favorites.) There are also a bunch of places to get hardware, most of whom visit this site regularly.
Don't worry about your ideas being crazy. You're way, way down on a long list in that respect.
