[mein12]

Hi Iain, you probably know this already, but the only way to tighten the belt is to remove shims. The larger OD crank pulley on the 68 is the one that drives the generator.
If this does not work it is possible that the rear generator pulley is not shimmed away from the generator pulley hub far enough.
When this happens the generator pulley hub will extend too far out and the nut will tighten against the hub instead of the pulley. It is almost invisible when it is real close. Make sure that you are tightening against the pullies and not the hub. I have seen this happen to lots of people. Good luck...JMB