Centre fitting fits cast head John, very little air resistance when clean anyway. Just had a look down the shed and think centre fixing would JUST fit an alloy head although you might have to put it on in bits,
I wont put mine on until I have the bike running right and then fit and see, Very easy to blow through but perhaps I don't blow as hard as an A10 sucks so maybe a drop on the needle needed and/or bigger cutaway, I'm no expert tuner though.
Ah Goldie got here first, good it goes with out alteration then