The Stovebolt Site Forums The Shop Area Driveline 55-59 3100 spindle adapter

It appears you are intending to use the 5 x 4 3/4" lug bolt pattern. That was only available on a passenger car. Truck spindles are longer therefor the aftermarket "kits" use a spacer behind the inner bearing to make up the difference. I doubt any of the aftermarket suppliers will sell you just the spacer or share any information on what to use. It's also not very likely that many of the 6 lug disc brake components can be used with the 5 lug kit. The mounting brackets are likely different which changes the location of the rotors and calipers relative to the spindle. The bearings could also be different depending on what rotors they provide. My suggestion would be to obtain a complete 5 lug kit, install it and try to sell your 6 lug parts.
Fred