GVW Rating of my GMC

[LongBox]

According to the plate on the LH doorpost, my 60 GMC 910 has a GVW rating off 5000 lbs. It has, according to the SPID, Heavy duty front and rear springs, a front anti sway bar, and 700 x 15 truck tires on 15 by 5.5 rims. The rear end is a 12 bolt, and it has genuine factory drum brakes all round.

It seems that the truck without a load or passengers would weigh somewhere right around 4000 lbs. Put in two passengers, and a full tank of gas, and your are down to a load capacity of about 600 lbs. Doesn't seem right to me.

Anybody got any information, or even opinions on the capacity of the axles, and the tires, so that I can figure out how much weight this old truck could carry?