After staring at the Vorshlag Posts and doing some searching. I do believe they are using CTS-V accessories with the LS3 Water Pump. Running a pulley on the lower CTS-V Alternator bracket using some washers/custom spacer and a custom bolt.
CTS-V Accessories from an LS Swap Miata:
(Not LS3 Water Pump, but note how the belt sits at the very back of the pulley)
Vorshlag:
Custom Spacer:
The Crank Pulley is a Cadillac Crank Pulley and the water pump is a LS3 pump.