This is the bracket that bolts to the block and is where you mount in the knock sensor. It is under the lower intake plenum, but goes over the water crossover pipe.
This part is the same for all 3000GT and Stealth 6G72 models, and is also the same on the DOHC 6G74 3.5-liter blocks.
Four bolts are sold separately that mount it to the block as pictured here: