Mike, welcome to PY!
Sounds like you've scored yourself a great (big!) cruiser. If the car has been cared for the way you say it should last a good long time. I'd be a little wary about driving that far for the maiden voyage just for that fact that it hasn't been driven much in a long time.
Pontiac motors will run forever if they're not abused (over revved), same for the TH400. You shouldn't have too much trouble finding basic maintainance parts for your car but body and trim is a whole other story.
That Grandville will be a dream to drive out on the road, the 455 should move it around with ease!
It won't be any canyon carver but it'll be competent in modern traffic and have a smoooooooooth ride.
Good luck with it and post some pics when you get it home.
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1966 GTO
1969 Lemans Convertible- F.A.S.T. legal family cruiser. 12.59 on G70-14 Polyglas tires. 1.78 60'
1969 Bonneville Safari- cross country family cruiser. .
1979 Trans Am 400, 4-speed, 4 wheel disc.
View from the drivers seat racing down Atco Raceway-
http://www.youtube.com/watch?v=GhYDMdOEC7A
Ride along in the other lane-
http://www.youtube.com/watch?v=mIzgpLtF_uw