I think this is what I want to do, but don't know the stock tank well enough. This is on my 03 2500HD.
I need to be able to over fill my gas tank (adding aux tank) and my plan is to plug all the holes in the stock tank. I can just run my vent stuff off the top of the new tank. My theory is to run the stock filler hose strait to the new tank. Then just plug all the other holes. (2 vents?) But will this work? I think the stock pump assembly seals to the tank pretty good. Where might leak? Is this a terrible idea?

 

Yes this sounds like big trouble. Easiest way is to mount a second tank, run a transfer pump from the new to original. Have the new tank pump fuel into the filler neck. Use a separate switch that you control. This will avoid evap leak codes and leaks.

 

Ditto.

 

Yeah, you guys are right. This has Wile E. Coyote written all over it. I'll wire up a pump.