If your asking if you can use SOHC heads on DOHC block and vice versa, the short answer is yes. There's always exceptions to things. How much smaller are the heads? Couldn't tell you, I've owned multiple 4v's and now own a 3v GT.. And I can tell you looking at it they do look smaller. But the amount may be deceiving because of the way the 05+ Engine Bay is setup.
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