Hi and Welcome to the board.
According to the Factory Assembly Manual I have your truck should be:
C = Conventional Chassis 4 x 2
S = V 6 Gas
1 = 1/2 ton
34 = Cab and pick up box ( Fleetside? )
- = no State Certification code
1 = Made in Oshawa
Z = 1970 15 Series
53376 = is the Unit made. 53,376th one made.
You will probably have to get the ID #'s off the engine to figure out what size it actually is.
GMC VINs don't cross over with Chevys from what I have figured out. Under Chevy, mine is a '64 Chevy not a '70 GMC.
Hope this helps. Also, do you have any pics of it? You'll see this alot on here.
Mike
