Lee-
On my '61 wagon, the two pulley assemblies are mounted to the firewall with two phillips head machine screws each. I wasn't sure how to adjust them either, so I pulled one of them out and had a look.
There are two pulleys on each assembly. One of the two pulleys is mounted on a spring loaded pivot, and there is a lockdown nut to keep it from moving once tightened. As mentioned above, there is a stop to limit the amount of movement in the spring loaded pulley. If you can't get enough travel out of the adjustment to get rid of all of the slack, I think you can grind off some of the stop to increase the adjustment travel.
While I had mine out of the wagon, I cleaned it up, lubed the pulleys, and got the adjustment working smoothly. Once I put it back in, and I didn't have to stand on my head to understand how to adjust it, it was easy to get the cables tight.
Hope this helps...
Pete