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The tube goes between the accessory bolt holes on the rear face of the heads. I believe it goes to either the smog pump and/or the cats.
The front and back of the heads both have that 5/8"-11 thd hole. The fronts usually get a 7/16 thread reducer to mount the front dress, if required. Ford used the air injection for many years , my '66' Cal motor has the injection right at the exhaust ports thru an external manifold.Secondary air injection. The smog pump pumps freash air into them which is inserted into the exhaust ports (the bumps that people always grind off) which then helps burn any excess fuel in the cats.
Kind related but not really... Did Ford always use those? My E6's took a big bolt (5/8 IIRC) to plug them up, my E7's have what looks like threaded inserts in them that appear to be plugged on at the end. Might take something like a 3/8 bolt inside the bushing?
On the front side of the head they have big plugs with a slot for a screwdriver...