I'm going through a struggle with those blocks myself right now. I,too, have a '48 half ton. I completely dissambled it many years ago and am fighting to get it together now. I had the bed on and the rear fenders on, but when I tried to put on the runningboards, nothing lined up. I adjusted the fenders, and even tried to shim the runninboards, but no luck. Finally, I cut down the blocks, and that lowered the bed enough so that it now lines up pretty well. I'll be putting on the runningboards next week.
Now to your question. The blocks I bought were 2.25" high, 1.75" wide, and 2.25" long. The rubber bushing adds just under a quarter inch to the assembled height. I had to cut the height of the blocks down to 1.75" to make the bed the correct height.
Oh, and the hole in the block is a half inch diameter.
