I believe the casting source for the 312 aluminum intake manifolds was Schmeller Aluminum Foundry in Cleveland. That name shows up in some of the paperwork. In addition, Schmeller was a source for other aluminum sand cast components to Ford.
A majority of the high performance FE engine series aluminum intake manifold castings was Buddy Bar. With Dearborn Steel Tubing Co. doing the machining and assy, as they also did on the high performance Y Block components.