The '18 carbs are almost identical to the later '21's.

I've run '18 carbs on '21's and there isn't anything in it.

'18 cylinders are better than '21/28 barrels port wise as stock though. '18 have a little more mid range and more top end. The boost port is better on the '18 and the exhaust port window is more performance orientated.

Of course the '21's can be modified to equal it.
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If I have to take the carbs off once more...