You're welcome Fourdy; sorry I do not have any pulleys for sale, those were old pics I took of a GMC collector's stash.
Here is a possible (?) alternative for adding more grooves to the crank. I noticed a 3-groove pulley, used on 401 Nail head Buicks and 455's, had "almost" the same bolt pattern as the GMC (minus two more holes). I took a shot and bought one to see if it would work. The Red Pulley is the factory single groove unit.
It's close but the bolts do not line up, still it's something to consider. Maybe tack weld or just drill out the holes? I don't know if the balance of the pulley would be problematic, and you would still have the double groove water pump pulley to find as well.
Of course, the Factory Pulleys are always preferred
I was just messing with the idea before I found a set of factory pulleys, so proceed at your own risk.