1974 W100 speedometer cable. WHAT???

I've been working on this truck from my dad, almost have the transmission ready to install, there is no drive pinion in the housing of the transmission tail. The cable is routed to a little black box along the frame and another cable comes out of the box and is routed to the transfer case. Has anyone ever seen this? Also, the speedometer wasn't working last time I drove it, it stayed at about 15 no matter what speed you were going, once in a while it would rise to about 22 and stay there but eventually it would settle back at around 15 while driving. Can I purchase a new pinion, install it and just route my cable there? I don't like not having a working speedometer, especially if my wife is also going to drive this truck. Also, the truck has 33 X 12.5 15 tires, is there a special pinion I would need to purchase for those size tires since they aren't stock to have an accurate reading? Thanks!