Frame I.D. Help!!!

[Slammed50]

I just purchased a donor chassis for the HD disc brakes and 5x5 bolt pattern but am curious as to how to decode the numbers stamped on the frame. Both the number under the left door and by the left motor mount say "C4Z 31774". I haven't been able to find out any thing searching the Internet so far that matches up with what numbers I have. I know it is a 73-87 chassis because of the disc brakes and rubber bushings in the front control arms, just not sure what year it is exactly. I am trying to gather more parts and just want to be sure I get the right stuff. Hopefully with all the "C10 heads" on here can help me out. Thanks.
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