Hi Tophat..
i changed my manual brakes to power braqkes 2...didn't change to disks yet but i have completed the hard part...
I took a booster from a dodge b250 van from 1984...i cut off the spindles from both manual and powerbooster..
I cut off the end of the spindle (app 1'' or 2'')....then placed the power booster in its location and i measured the space between the cut off spindle and brakepad...watch out, be sure that the spindle can be pushed all the way inside the booster..or else the brakes won't work.
After that, i cut of the original spindle to its exact length, grind the axle to a point (one side only) and welded it back together...because the spindle doesn't have any sideways pressure the weld is save from braking...if you weld the right way off course

....
If the booster doesn't fit, maybe you can try a booster from a volvo...i had one on my 1971 dodge truck, 4×4 with drum brakes..and it worked pretty well...only problem i don't know which year the booster was from...maybe between 80 and 90's....
Good luck m8....
greetz Roger