Are you sure that you connected to wire from the ignition switch to the correct terminal on the starter solenoid?
There are two small studs on the starter, one on the block side and one on the fender side. If i remember right the trigger wire from the ignition switch must be connected to the stud on the block side of the starter solenoid.
The other stud is only used for vehicles with points. It provides 12 volts to the points while cranking. The rest of the time the points get 6v from the ballast resistor.
You need one wire (it will have a large ring terminal on it) on the same terminal as the battery cable. It sounds as tho this is connected as you have power to the rest of the truck.
That means either the ignition switch isnt working, or the circuit is not being completed due to a misadjusted or faulty neutral safety switch. Under the dash i believe the wire from the ignition switch to the neutral safety switch is yellow and the wire from the safety switch to the starter is purple. Start at the starter and work your way back if you determine that you arent getting power from the ignition switch. Remember purple and yellow wires (atleast thats what color they are on my 79 C-10) and they are large, #12 or #10 size.