yes 20 series is a 3/4 ton 8 lug wheel
lb is 8 feet, sb is 6 feet inside; outside they are about 7 inches longer than inside each.
it is pretty easy to switch the 1/2 ton stuff to the 3/4 ton frame, that is if your cab vin and frame vin match.. it is even easier to bolt a good 3/4 ton cab with a title to a swb frame if that is what one wants.
a couple scenarios of what might have happened with your truck.
check out this thread to help find the vin that should be stamped on your frame:
http://67-72chevytrucks.com/vboard/s...d.php?t=316902