I would think that a simple momentary contact switch to ground on the left side of the dash would be easy. That way you can give it glow while cranking over. Unless you have two right arms...
Is your system not working correctly? I wouldn't say the system is poorly designed or troublesome, just remember that these things are 30 years old. You may need a new card, temp sensor, or GP's. Everything needs to be fixed at some point. Mine has been trouble free for 3 years after getting the bugs worked out. If you really are dead set of gutting the system, use the solenoids ground wire going to the controller on a momentary contact switch and call it good. No reason to reinvent the wheel or use some ridiculously oversized switch on the dash and have the GP power going into the cab.