All that will do is slow the fan down too much and over heat the heads
![Roll Eyes](http://cal-look.no/lounge/Smileys/default/rolleyes.gif)
which are probably already suffering with the extra under decklid heat your turbo kit adds
I think a stock 68 4.12 trans will probably have a .82 4th in it already so the 3.88 would seem your best option, but if its just a street car then look at pulley speed as well, if you gear it taller then maybe speed the fan back up with the Porsche top pulley swap[ I have never done this pulley swap but have heard good things from some who have]
cheers Richie