proportion valve and strange cable question

[btorear]

I have a 71 3/4 ton 4x4 with a 396. Anyway, i can't find a proportioning valve for it anywhere. Everytime I find one that looks like it'll work it turns out to only be for a 1/2 ton. Any ideas where i could find something that'll work?
Also, there's a cable that looks like it should be hooked up to the throttle but it runs down to the transmission. It's been disconnected since I bought the truck (i am a novice, but learning new stuff all the time) what is this called? And it looks worn out...where can I find a new one. I heard that this cable has something to do with the truck goin intopassing gear when the accellerator is pressed down (which my truck doesnt do. That's another thing about this truck, it has a 396 big block but feels like it has no real power at all). Any help would be appreciated.
Thanks