Power discs suit one piece cranks or tubular cro-mo cranks like 401s or Profiles.
401s need a shim to use a power disc as the 401 spindle has a smaller diameter.
You can't run one with a 3-piece as the spider is built on to the drive arm.
Suntour VX cranks in 175mm:

Sugino Super Maxy in 170mm:

180mm 401s with a Pro Neck power disc:

180mm 2nd gen Profiles with a Pro Neck power disc:

Sugino 175mm one-piece with a TufNeck power disc:

SR 180mm Tubular one-piece withw a TufNeck power disc: