All that will do is slow the fan down too much and over heat the heads
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