Just a quick photo dump to vaguely document one way to get a disc brake and high mids on an 17mm axle ironhead on the cheap. Wheel is a 3/4 axle dual flange. New axle is 3/4 dia from jenna's unused '81 ironhead swingarm(length unchanged). Caliper is the 4 piston radial flavor off a gsxr(108 mm bolt spacing). Rotor is a drilled, floating, 11.5" hd. Master cylinder is a 19mm Grimeca.
To use the 3/4 axle bore out the chain adjusters, the axle collar(stepped washer thing), and buy an extra axle collar for the other side and bore it out. Turn the chain side wheel spacer to whatever length centers the wheel in the frame. Measure what's left for a wheel spacer on the brake side and then subtract the thickness of your caliper bracket. I had to make a .120 thk washer to move the bracket away from the chain adjuster bushing a little. Last figure out what you need for a sprocket for a straight chainline. In my case I used a 48t, 1/8" offset.