[18436572GMC]

I agree with everyone else on what you have. A 67-72 frame due to the fact that the body mounts line up, np-205 t case, dana 44 front, eaton ho-52 or ho-72 (ho-72 has load bolt on side of housing) I did see a sign not mentioned that your axle has been changed. look at how far the u bolts stick up past the nuts, they are longer than factory u bolts.
As for a rear disc brake conversion I use ruff stuff specialties 14 bolt brackets. You will need to use 72 front calipers pads and rotors, and drill out the wheel stud holes to fit 9/16th studs. I used wheel studs for a 1995 f250. 72 front brake lines will work to connect to hard lines. I run this with no proportioning valve and it does great.
didn't mean to make that so long.