[67 C-10 LWB]

Since you said you have 6 lug axles you most likely have a 1/2 ton, unless someone converted it to 6 lug. Plus you said it's a short bed, there were no actual 3/4 ton or higher short beds. So someone would have had to chop the frame and put on a short bed and convert it to 6 lug IF it was originally 3/4 ton.

You most likely have a 1/2 ton. Also there may be badges on the side that should say something like C-10 (1/2 ton) or C-20 (3/4 ton).

Hope this helps.