anything over 15v is a redflag
it sounds to me as if there may be a voltage regulator issue in the alternator, possibly a diode problem. have the alternator tested and see. mrfixit's suggestion sounds right on about checking the "runtime" voltage. It should be anywhere from 13.2v to 14.8v. One other thing i would check would be the positive output wire from alternator to battery, check for chafing or rubbing to frame and if it has fuselinks check those too. (not fuses but fuselinks, they are special wire links that are designed to blow under overload conditions)

as for the high resistance in the key switch? thats not uncommon if it has age on it, replacing it might be a plus but i doubt it can cause enough resistance to blow a 40A fuse without a noticable immediate problem when the truck is running. The only thing i can think of that it may cause would be a meltdown situation in the switch itself from the heat transfer inside the switch area from the bad connection.