Welcome to the Big Bolt Group!
I don't see that anyone decoded your vin number for you so I will...
6B59O104699
6B = 6400 Series (156.5" wheelbase)
59 = 1959
O = Oakland Assembly Plant
104699 = Unit number from Oakland Plant starting with 100001
Looks like your 1958 is really a 1959...
Your truck should have the 261 cu. in, 6 cyl motor with the SM-420 transmission.
If you have 20" tires, DO NOT PUT AIR IN THEM until you have identified what type of rim you have. If they are two-piece with the center lock band they are dangerous. If they are 3-piece they can be services, put find someone that has experience with truck tires check them first. Many people have been killed by the old rusty rims flying apart when they are aired up. Just be safe.
Mike B
