Isuzu's added in a mechanical brake vacuum pump on some of the 18000 GVW applications. IIRC, they used a different ps pump and generator bracket too, but I am not 100% sure on that. Either way, you changeover to a regular FST accessory drive. And you can run a regular PCM harness, you might even be able to adapt that one, IIRC, they used the GM PCM too.

I wish I had a pic, but I can't find one right now.