First off, the 6G72 DOHC engine in the 3000GT is not at all a direct swap for our cars - if you have ever seen one before, you'd know it's mounted opposite our engines. It just doesn't have an extra cam - there are 2 extra cams (one on each bank). You clearly don't understand how an OHC engine works if you are asking what the extra cam does. One cam controls intake valve timing, the other controls exhaust valve timing. The differences are greater than that - the ports are larger on the 6G72 DOHC as are the valve heads. I think it's in your best interest to just keep the 4G64 and not worry about swapping in a V6 (although it is possible to swap in the SOHC 24V 6G72, it's definitely not worth it).
A lot of the answers you are looking for can be found by searching (here and on Club3G).
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