you can get rebuild kits for the calipers on rockauto and ebay.
Didn't the 620 calipers have a short hard line connected directly to them.....accounting for the female end on the rubber hose.
If you're going to spend some money on that front end, my suggestion would be to get spindles from a D21 Hardbody.... '86 and later. You get better calipers, vented rotors. You can use the hubs you have with the HB spindle. I'd avoid any spindle earlier than that because of wheel fitment. The swap is just disconnecting the upper and lower ball joints, tierod end and brake hose from the hardline at the body.
Getting the rotors off....provided you're not going to reuse them(vid). If you're going to have them turned, put the bolts back in and don't try to remove the rotors.
More info about the brake differences for scrapyard reference....
http://www.bluehandsvideo.com/hub-info.html
If you haven't R&R'd bearings before, these vids should get you started....
More pics of your suspension might help solve the confusion over your frame/frontend. There have been cases where guys have bolted the disc arms to the drum frame. The shock tower on the drum trucks were inline with the frame. The towers on the disc trucks were at an angle. I'm not that familiar with the 620 disc frames to recognize if the entire frame has been swapped.
Good luck