What size clutch head bolts?

[vetnat]

Hi. I recently acquired this truck, mostly because its in pretty nice shape, and I wanted the fender with the spare tire cut out. It turns out the truck was originally a fleetside, but some PO took that bed and put on a stepside bed. The truck is a 1961 GMC 910(?), a Canadian half ton. The bed appears to be from a Chevy. In real life, the colour of the cab and bed are much closer than they appear in this picture.

The spare tire holder is held on with clutch head bolts. The fender itself is held on with phillips head bolts. Not quite sure what year the bed is? Anyone know what size the clutch head bolts are? I need to order a clutch head socket to take it off. The truck is parked at my parents farm, about 2 hours away from me. I was just out there today and forgot to measure the bolts!