Hi 54barnfind, I'm pretty sure that is part of the pollution control control system that went to/fed an air control valve. You are right at the end of 250 production. Does your engine have the 2bbl intake and dual exhaust, and the valve cover with the two humps? Find a Chilton's or Glenn's manual and look in emission controls. Because you have the "one piece" head, that thing probably goes right into the intake! When I put a replacement engine in my '75 C10 it was a late "pollution" engine with all the stuff. Because it wasn't required, I pulled it all and replaced the carb with a Webber 2bbl, and plugged all the extra holes. It made a runnin' 6.
best wishes,
Les
