If you choose to go with the key-start system any universal type ignition switch with a spring-loaded start position can be made to work with the solenoid-engaged starter. The original equipnment ignition switch for a passenger car would work, if you're interested in keeping things all-GM, but that's not really necessary. Another option would be to keep the truck ignition switch and just install a pushbutton switch to engage the starter, like the Hydramatic-equipped trucks did. Those switches mounted on the left side of the dash, just above the parking brake release knob.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!