I have an '05 Duramax and I am getting an auxiliary fuel tank built, but I don't know what all I am going to need to install it. What is the best way to connect to the main tank? What parts am I going to need to make this work?

Lots of ways to do it. I hoked a simple electric lift pump to mine and plumbed the discharge into the vent tube. This is slow and cannot be left unattended, but effective. My plan however is to add a delivery pump to the tank with a flow meter. Then I can measure fuel pumped into the main tank for better mpg calculations as well as fueling my tractor(s).

http://www2.catalognavigator.com/pollak/viewitems/fuel-tank-selector-valves/light-truck-6-port-motor-driven-valve?&plpver=1002&forward=1&backtoname=&pane=

I'll also suggest adding a filter to the aux tank at least for the first few loads. I pumped mine through a small filter and after the first 90+ gallons it was pretty tough to push through.