Aren't all 3/4 tons longbeds?
GMC made a 1/2-ton long box truck, and the GMC serial number listing include 1-ton trucks in the 3/4-ton length.
So while as far as I know all 3/4-tons Chevy trucks of the AD era are either long box or flat bed, a long-box does not definitively mean the truck underneath was born a 3/4-ton.
The SM420 was not exclusive to any particular size of truck.
