The intake and exhaust port spacing is different between the water cooled and oil cooled engines.
#2 & #3 will present more of a challenge than the outer cylinders.
Its all been done before, but some systems will be easier. For example, D&D headers have separate head pipes that slip into the collector...these give you some flexibility as you can adjust them to fit the oilers. So you could look out for a set of those or something similar.
Carbs can be "re-racked" and exhaust tubes can be heated up and bent. Anything is possible.
Some guys have even finessed late model titanium systems.
Of course oil pan clearance will be an issue too.
I wouldn't go out and buy a really nice water cooled system, but if you had access to a cheap one or a freebie...than what have you got to lose?
Nothing ventured, nothing gained.
