The difference between the drum spindle and disc spindle is one bolt - the top pivot/shoe stop (drum) bolt size and boss, which becomes the new caliper bracket bolt hole, plus the part number. If you can get a machine shop to mill the boss down, and retap the bolt thread, the spindle will bolt right back up and will work with factory components. I've known several people that have done it. Another option would be to use an alternative GM body spindle (there are actually a number of exchange vehicles that use the same spindles) - check a Hollander manual. Several of these might use a different steering geometry, and might require either steering arm reversals, or the use of Camaro steering arms as a worst case (still cheaper than buying new). Boneyards are cheap sources.
The only thing I wouldn't trust is an import knock-off - if you buy new, try to make sure it's USA made, rather than trust your ride and your life to an unknown -