Or you could install a redundant switch in the starter control circuit.
Just cut into the purple wire that comes from the ignition switch and goes to the starter solenoid. That wire carries no real current. It just supplies enough to pull the solenoid in - after that the main cable from the battery cranks the starter.
Drill a neat hole on the lower part of the dash under and to the left of the steering column. Mount a momentary type push-button switch in the hole. I recently used this switch (Advance Auto $8.39) for a similar modification - works/looks great:
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Attach two leads (maybe 3 feet long) to the terminals.
Find a convenient place to cut into the purple (ignition switch to solenoid) wire. Cut it, strip the ends and splice the leads from the momentary switch in.
Now, with this switch in the circuit, to start the car requires two hands. Using your right hand turn the ignition switch to START. At the same time, using your left hand, reach under the dash and push the redundant switch in.
It's kind of like launching a ballistic missile.
BTW - there is no neutral safety switch in your truck. Although people talk about NSS's all the time, they were long gone by 1985. The electrical switch was phased out and replaced with a mechanical interlock within the steering column. Try twisting the key to START with the gear shift selector in any position other than P/N. You can't. Look down at the base of the column and see the NSS bracket. All you'll find are the leads for the back-up lights.
