I encountered the same problem with a 65 C-20.
If you were able to remove the bolts from the rear wheel cylinders in fairly good order, use them on the fronts. You should be able to use a standard 5/16 bolt on the rears as there is more room.
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