Pulling the negative battery cable isn't going to do it because the engine isn't grounded through the battery - it has its own ground.
I'd start by inspection the ignition switch itself, as well as the accompanying wire harness for any shorts... any breaks, fraying in the wire, peeling of the insulation, etc.
http://wiki.answers.com/Q/Where_can_you_find_the_wiring_diagram_for_a_1982_ford_e3501982 ford e350
your local ford dealership
Lower rear end of engine - follow the + battery cable
Facing the engine compartment, on the left hand side directly in front of the battery attached to the front wall. Follow the large cable from the positive battery cable and you will find it.
You can find a fuse box diagram for a 1985 Ford E350 7.5 liter inside the owner's manual. You can also find it in the vehicle's Chilton repair manual.
In the glove box...
Unhook the cable from the Cavaliers battery. Find the opposite end of the cable and pull it out. Fit the new cable in following the old cables route. Connect the non battery end of the cable, then hook to the battery.
In bed with britney spears
go to your nearest ford dealer or look it up on line
its hiden under the air filter you have to take alot of stuff off if its a 351 ford
It should be in the engine compartment somewhere close to the battery. The easiest way to find it is to follow the positive (red) cable from the battery until it connects to the solenoid. The solenoid is not very big and also has one or two small wires connected to it besides the battery cable.