2006 is going to be an ASET AC-xxx (xxx is horsepower rating) with an external EGR. The AC motor was used on highway chassis, basically everything other than the Granite, RD, and the trash trucks. Those got the AI-xxx motors, which used an extra bump on the exhaust loves of the camshaft as a rudimentary EGR.
The EECU, the engine mounted computer, on the AC uses 4 electrical plugs I believe, whereas the AI and ETECH use 3 plugs. ETECH left in around 2002 and the ASET engines started in 2003. I wouldn't junk the truck, but I'd look for an AC motor to drop in.
Alternatively, if you have a manual transmission you could try and find a donor Granite or trash truck and swap in all the electronics, wiring, etc to convert the truck to an AI configuration, which is much more user friendly.