If I am not mistaken , that should be the same manifold that is on a 390 . Your casting # shows that it was originally for a 63 engine . In 63 the 390 would have been the only engine that this # would work on .
These are readily available on ebay . The left side is much easier to find than the right as the right side tended to crack. Should be in the $100 range .
My parts manual shows, for 66 428, RH-C6AZ-9430-A and Left hand-C3AZ-9431-D. I’m having trouble with my right had manifold and have found several on ebay but they are showing a Galaxie part #. I’m told that the problem with older exhaust manifolds is that too often they are either cracked or warped. Warped is my problem. I have noticed a couple NOS and aftermarket right hand manifolds too. They are pretty expensive.