1972 Blazer Underhood Wiring

[tim71]

I'm trying to get the wiring reinstalled in the 72 Blazer build that I'm working on. A PO had molested all the wiring harnesses. I also have a 71 GMC K1500 that I refer to since both the truck and Blazer have 350/350 with AC.
My 1971 GMC has two relays (tcs and emission) on the engine side of firewall just above and to the right of the fuse panel. The Blazer doesn't have these relays. Does anyone know, is this a Blazer thing or a 1972 thing. I want to find a underhood harness but don't know if the harness would have to come out of a Blazer, or could it be out of a 1972 C or K truck also.