Alright, as you look at the pick of the top of the gearbox you will see the bolt and the screw in the middle of it. What you want to do is put a large srew driver in the screw and while holding it still lossen the bolt around the screw. Once you get the bolt a little loose, then tighten the screw a 1/4 turn and then tighten the bolt back up while holding the screw so it does not try to tighten also. It will try to tighten at the same time and you only want to adjust the box 1/4 turn at a time. Once you get them tight again, take the truck out around the block and see how much slop you still have, and adjust it again 1/4 turn if you need to. I am not too sure how far you can turn it before you should not anymore, but i would only turn 1/2 total for right now if you need to turn it that far. If you turn it too far it will start to bind up inside and can break one of the gears in there.
Spray some PB blaster or WD 40 on it before you try to play with it.
Did it help you ? Also before i would put it in FAQ i would like to do more research and see how much you can actually turn them in, because i know if you turn them too much they can break when you are driving, that is why you only turn a little at a time, just to take the slack out. I have rebuilt one of these also and it was not too hard If its on a cruiser i have probably rebuilt it once or twice now. I am cheap, and if it can be rebuilt i will do it, and hope it works. I can even tell you how far to go on a carb rebuild, and what not to take apart on it. Don't ask why .