I think you are right about the spacer. Years ago I bought a bunch of stuff from a speedshop that was closing and among it was a disc brake conversion kit for early Ford spindles. There were two cylindrical spacers in the box that you slid onto the spindles first, and they had a raised portion toward the inside so that the bearing would come up against it and stop, effectively shortening the spindle to the correct dimensions of the rotor.

Don