UPDATE*
just to make sure that everyone is on the same page...
i have 1969 datsun with the beebani disc brake conversion. my wheels were rubbing on the hub and not sitting flush against the disc brake, so i ordered 1/4 spacers...
the 1/4 spacers arrived today. i put them on the 521 and found out the following:
~the spacers center bore was plenty big and fit over the protruding hub with out touching the hub, which is good so you wouldnt have to machine the hub.
~the rim's center cap still doesnt fit with the dust cap on, but does fit with the dust cap off. so maybe you could trim the cap and reweld it so that it fits under it or even attempt to beat the top of the cap flat with a hammer until its more flush.
~the stock open ended lug nuts are only about half way threaded on to the lug bolt fully tightened. it was expected but i was curious. some people might not have a problem running them like this but i think its unsafe and i wont risk getting injured just to make the truck look better. to remedy this you would simply have to get extended lug bolts.
but for me, ill be machining the hubs and running them without the spacers or center caps, since i already bought the wheels i might as well use them. i might end up doing a 5 lug conversion by getting the hubs and axles filled in and redrilled with a 5 lug pattern.
but that is to be decided at a later date. i still have a few other things i need to button up such as tucking my exhaust and coilovers as well as a few other things on a much larger scale.