I just found a thread on "Bed Mounting Blocks." The suggested dimension is 2 1/4 x 2 1/4 x 2 1/4. That sounds about right.

The following site/thread gives instuctions and parts for bed mounting. It's a bit later model pickup, but should be close enough for government work. Also, the instructions do include and isolation pad between the frame and bed block. I'm using belting that's about 3/8", and will continue with that. It should be close enough to the cab height. I probably will re-use the metal blocks in place of the wood.

http://www.oldcarmanualproject.com/articles/TIPS/ChevTruckTIPS/Tip103A.pdf

Thanks all,

Gary