wachs, I agree with what Neill has said about the GTA. The boys that were young and taking an interest in cars back then are hitting their 40's now and will be looking for these cars.
Regarding the regret if you sell it? I expect you would, especially such a nice example.
On to the Sprint. I wouldn't worry about extending the time - especially if you have the GTA. If you are concerned about mechanical parts availabilty start by stock piling those now. If you are mechanically inclined look around for a technical school that offers auto body classes and have a stab at doing it up right yourself.
Cal