More disc brake questions

Ive searched, and im still a little unclear, so i was hoping you guys could help. I just bought a roller 73 dart sport, /6, 3 on the tree, and the previous owner did the OEM disc brake swap. Yay! Except its still on the small bolt pattern, so my wheels wont work.

This will be a 95% track, 5% street car, so im ok selling the OEM setup, and going to some Wilwoods or Stranges. But it looks like they are only made for the drum spindle. Is my only option to change out the disc spindle too? And how will i know if i have the correct balljoint setup? Again, being a drag car, i want my front end to move with the least amount of resistence without the shock.

Thanks!