1/2 tons should have 6 bolt wheels and 3/4 and 1 ton should have 8 bolt wheels.

Once you get there you should be able to look at the serial number and get the model from that. Here is a link to decode a Canadian GMC as an example.

http://www.chevytrucks.org/tech/canadavin.jpg


If this truck is an American Chevrolet take a look at this link.
http://www.stovebolt.com/techtips/series.htm

Or for a more detailed breakdown take a look at

http://www.chevytrucks.org/mikesgarage/1958.htm
http://www.chevytrucks.org/mikesgarage/1959.htm

Blaine Dumkee
59 GMC 9314
Fort Smith NT
www.flickr.com/photos/northerngmc/