Let me start with saying that before I found this webpage, I knew nothing about working on cars - not much of a gearhead, but wish I were! Learned a TON about these cars just by reading up here, and posting some questions. I bought my '68 a couple years ago just "blindly" without knowing anything about them, except they looked cool. Hey, I know what I like!
Anyways - I was wondering if you could post some pics of the car in it's current condition. That may help some of the knowledgable guys here decide better what is your best route to go. If the thing is a rotten heap, then it may take a ton of work just to be on the road SAFELY. However, if it's just some rust on some of the body panels, and nothing structural (solid frame, etc) - then you might be able to do the mechanicals first and get the car on the road, and then work on the body.
You didn't mention - how is the interior?
Did you park the car over grass for 13 years, or pavement? If you parked it over anything "organic", then it's very possible your floors are rusted out too. You'll have to rip out the entire interior and replace the floors and carpeting.
As for frame-off restoration vs other methods - it just depends on your time frame and your end result. If you just want a presentable driver to have fun with again, then you might be able to skip the frame off job, assuming everything is safe. Also, depending on how much time or spare cash you have will depend on when you can start driving the car again. Doing the work yourself, you can save a bunch of money - but it could also take a few years of constant work (evenings, weekends, etc). And you'll need a 2 car garage to work in during that entire time. (or a large 1 car garage) Getting the work done by a shop might be quicker, but could still take a year, depending on what needs to be done, and the level of the work. Also, costs end up multiplying from the original restoration quote as more things pop up. It just happens.
Again - if you could post pics of the body, interior, engine, and undercarriage that would be helpful so we can help guide you in a direction.
Welcome to the board!!
'68 Firebird, 350-4, 2 spd auto, triple black, Dlx Interior