If you are looking to go to the five lug rear on your 66, get a 71-72 rear. It will bolt up without ANY modifications. Most people don't know this , but the frame for the 63 up to 72 chevy trucks were essentially the same - they just put a different body style on it 67 and after. This leads into your second question - if you get a 67-72 truck with disk brakes, the power booster will bolt right up - be sure to get the actuator rod though. And, yes it is possible to mount up the disk brakes to you 66 from a 67-72, but you'll need to do a relative few modifications. It is also possible to mount up 73-87 parts onto the 66, but there is more involved. Search through old posts on the subject - it's been beaten to death many times.
