[BOZO]

If you want to keep your 6 lug wheels and not have to change your rear end over, a good junk yard swap is converting to the later light 3/4 ton setup. I copied the following info info from this site a while back and if I'd known then what I know now I would have gone this route.

((Swap 6 bolt drum brakes over to 6 bolt disc brakes by using the front pieces off a 1988 Chevrolet pickup. Chevy made 6 lug 2wd 3/4ton trucks from 88 on. The 88 spindle, rotor, calipers, ball joints, and outer tie rods are used. Replace bottom a-arm from a 67 to 70 1/2 ton to a 67 to 70 3/4 ton a-arm. The top ball joint is the same but the bottom one has to be changed to an 88 ball joint. It pops right into the 3/4 ton a-arm. The top 88 ball joint fits into the upper 1/2 ton 69 a-arm, and the bottom 88 ball joint fits on the 67 to 70 ¾ ton a-arm. The top 88 ball joint you have to grind the rivets off of the 88 a-arm and then bolt it to the 69 a-arm.
Use the 69 intermediate shaft with the 88 outer tie rods. The inside tie rod end is changed to PART# : MOOG ES2020RL.(tie rod end). The 88 tie rods are also bulkier than the original 68 setup.))
As far as a good (and cheap) junk yard power brake setup, the Chevy Astro hydroboost unit is the way to go. You can look on this forum for more info on the hydroboost and the 6 lug disk brake swap.