This reply probably comes too late, but I wanted to point out that the '68 Mustang gear box IS compatible in a 1965-66 Mustang. All you have to do is add a rag joint. The best way to do this is to install a 1968 steering column which has the added safety advantage of being collapsible. I have pictures of this installation if you are interested. I don't believe power brakes are necessary. I have converted to '68 Mustang single piston disc on my 1966 Mustang and the manual foot pressure needed is very easy to apply.