From what I understand it takes an adapter plate to bolt it to the clutch housing. Floyd Isabell in PA sells them. I think alot of guys who do the swap build a custom frontend because of interference problems with the crank pulley. On the manifold I think Isabell sells an adapter to flip the down draft manifold over. I'm not an IH guy this info is all just what I've heard.
1966-72 406's were 193hp @ 3200rpm
1973-74 406's were 160hp @ 2700rpm
The engine I had for sale is a 1969 model.