honestly, I cannot imagine it is worth the trouble of those motors. You would have to get your hands on a australian head that has the intake manifold seperate from the head. As you have probably already seen, the stock intake manifold has huge distribution issues. I have seen one guy over on stangnet that cut out and tap/drilled to put 3 2bbls on it. One where the carb sits now and the two others at the ends (#1 cylinder and #6).
I am not sure on the pistons as I thought the 200s had a 3.68" bore. I believe 255s were 3.68" and the 2.3s are 3.8"
Probably one of those that is better to swap in a 302. I ditched my 200 that was in my 66 falcon because I got tired of flooring it to get out into traffic.