Most would presume that a question regarding a 58 Chevy truck with a floor starter would have a 235 or 261 Chevy engine unless it's stated in the question. It's possible that the same starter is used in the GMC 6 cylinder engines. I don't know, possibly someone else has some insight on this.
Fred

1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine