1) The difference may be that they started putting 250ci I6's in mustangs I know starting in 69 (maybe some in 68). Really should matter, IIRC, the manifold are interchangable. They may have some smog stuff that are different.
2) IIRC, $60-80 should be a reasonable price for a decent used one. MU has new ones for $120, but that seems a little bit high to me.
#1 Looking at NPD, they show the same manifold from 65-Nov 66 (mid 67 model year) where a C4 tranny and IMCO emissions took a different manifold. Should be able to used it on any, keeping in mind the later one has a 2" outlet and the early ones have a 1 7/8".
#2 When I was searching for a new one for me, the junkyards I called around to quoted between $50-75. The $50 was at a pull-it-yourself place, the engine was already in the warehouse from the owner's I-6 to V-8 swap. I pulled it off and was about to pay for it when I found a crack in one of the ports ::
I have an exhaust manifold off a 1965 Mustang 200CId 6 cylinder. I am selling it. I live in pensacola FL, 32514.
It has been painted blue with a high tempature paint. and the surface between the exhaust manifold and the head has been milled flat to insure a good tight seal.
Here is a picture