To get the 6 bolt rear axles redrilled properly for the most popular Chevy bolt pattern that most of the disc brake conversions use,, two adjacent bolt holes must be welded up, and the flanges need to be machined true again. It's not terribly difficult, but it does require some skillful welding and machining. IMHO, that procedure is MUCH better than using any sort of adapter. I'd also suggest attaching the brake drum to the axle flange with a couple of 5/16" countersunk flathead screws, as redrilling them to match the 5-bolt pattern compromises the matchup between the studs and the drum center plate.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!