I'm in the process of doing my cummins swap homework (so I'm following this thread closely) and found a couple guidelines for serpentine setups. One, you should try to have at least 1/3 of the accessory pulley wrapped by the belt. This wouldn't apply to idlers or tensioners since they're not driving anything. And I don't know if you can have too much wrap on a pulley. Second, the tensioner should be put at close to the "push" side of the crank pulley as possible since this is where you'd have the most slack in the belt. So with your current setup, I'd be worried about not having enough pull on the water pump and running into overheating issues. Not sure if you could relocate the tensioner between the alternator pulley and the crank pulley, then run the belt (going counter-clockwise) from the crank pulley, over the fan pulley, under the water pump pulley, then around the A/C compressor, idler pulley, alternator pulley, and finally along the tensioner. Just a thought.
On a separate note, how will you be controlling the electric fans? I believe GM had PCM's turning them on low speed at 202 F and high speed at 220 F, which is higher than the cummins should be running.