Czechman, I am not sure who you bought your disc brake upgrade kit from but I seem to recall most vendors used to mention that they didn't work with stock wheels as you discovered. Classic Performance Parts still states this as an issue:
http://www.classicperform.com/Store/1947_59_Trucks/4954CBK6.htm The latest Chevy Duty/Classic Parts catalog I have also states the same thing.
The Jim Carter (Old Chevy Trucks) catalog doesn't state it specifically in the section for the disc brake kit, but lists a 3/16" spacer right underneath the listing that solves the problem:
http://www.oldchevytrucks.biz/47_55/full.asp?page=56 Obviously this doesn't help you though at this point with having been misled but might guide you to who you can trust when buying the next major part.
Also, if you haven't bought/mounted the new wheels yet you might want to consider that Jim Carter part:
