1971 demon rear disc brakes

Hopefully I am posting this is the correct place; I got scolded by the forum police last time I posted in the wrong place.

I am shopping for a rear disc brake conversion for the 8 3/4 I am installing in my car, the rear appears to be 68-70 B body. Prices range from $400-$600 and can sky rocket from there. I am replacing all moving parts in the rear, have Dr Diff axles with green bearings and there is a small centering flange for the rotors (appx 2.8" OD). A couple of brake suppliers tell you to turn down this flange to under 2.75, this sounds like an assbackwards way to install rotors, universal kit gone wild.
I'm not rich and am doing 99% of the work myself so am requesting feedback on the most bang for the buck, but don't want a nightmare for my kids/grandkids when I'm long gone to find replacements either.
Thanks in advance for all the assistance.
Keith