You do have the correct wiper motor for 58-59 Chevy Pickups. It was the same motor used in 58 Chevy Cars but with extra brackets for the pickup which you should have . I hope that you bought the correct wiper linkage as the vacuum and earlier electrics used a different linkage. I am enclosing a link for bushings for the motor you have. I have never bought them but should work providing the outside diameter or the rubber is correct for your truck linkage. If they have good customer service they can give you a measurement. Thanks , good luck.
http://www.lategreatchevy.com/full-s...eals-1958.html