6 lug disc brakes onto a 1967 C20

[1967Bowtie]

I have read through a lot of threads on the disc brake conversions but I have just one question. I have a 1967 C20 and want the 6 lug setup, I read that to do this on a 1/2 ton you use 1988 truck parts and a 3/4 ton lower a arm. I have the lower a arm on my truck already being a c20 so I need a 1/2 upper arm. What years can I use for the 1/2 ton a arm is it only 1967-72 or van I use them from a 73-87 or the 1988. Or can I just use my 3/4 ton upper arm. Can someone point me in the right direction, I don't want to buy an aftermarket kit.