Goodyear Gatorback is the ONLY belt that doesn't occasionally squeal on the Duramax's. Best place I found was Amazon. There are lots of different sizes for the different series Duramax, different alternator combos, etc, so make sure you get the right one. Not a peep since I put mine on.
The Dmax loves bio-fuel (and any other fuel that manages to actually provide lubrication, like diesel is supposed to). Probably not absolutely necessary to run additive with it, but can't hurt.
I don't remember if you have or not, but other things that some people forget to change out on new-to-them rigs are transmission filter and fluid (allison has both the spin-on and an internal. not a very difficult job. make sure to use Transyn-D or compatible fluids. Aliisons are very picky on fluid. I use Amsoil with great results), front and rear diff fluid (changed mine out at 108k when I bought it. Made an improvement in mileage, and seemed to roll easier), transfer case fluid (4wd engages much quicker), power steering fluid (we have hydroboost systems, and it gets burnt up after a while). Of course, coolant, brake fluid, oil, air filter, fuel filter, etc.