CC,
Don't know if D6B (D333)/(D330) parts are the answer.
May be worth investing some time in the taper lock stuff as eventually these goofy adjustable sheaves are going to all wear out and need replacement with something.

Only seen one of the double v-belt drives on the D-318 and this is it:)

First of all I want to thank everyone for all the info. My D318's waterpump is gear driven. There is nothing driven off of the front of the crank, however they did offer pto's that had a hub (part # 4F8287) that went on the crank and then bolted onto the pto. I have had no luck locating one of these hubs. My engine also has the "auxillary power take off" with a double v-belt pulley. This arrangement is driven off of the front of the circulating water pump, and was intended to run things like a bilge pump. I want to drive a 24V 125 amp alternator and 24V alternators need 3 to 5 thousand rpms and requires 5-7 hp. A ten inch pulley on the crank and a 3 inch pulley on the alternator would be just right. I like the idea of finding a pulley and just using the hub, and machine a new belt face to suit my needs, anyone have a spare?