any one know of a part or a mod to make the filler neck bigger.
I just got a splash of diesel on my shoes (again) while fuelling up at the big truck's pump.

Supertanks or Transferflow sell them. Do a search on those along with "filler neck"....you should find your answer.

For the Lazy

http://www.transferflow.com/html/gm_fillneck_kits.html

If your handy, fix the existing one yourself. Best bet, remove screws
inside the filler door, and remove the hose on the bottom side going to the
tank. Now you can pull that part out, and use an airchisel, or a hammer
and chisel and open up your existing filler neck. clean up the filler neck
you just hammered on, and put it back together. Mine took less than a
half hour from start to finnish. Those big truck diesel pumps nozzles now
fit inside, and let me fill without a spill. Good luck.
Note: I do not recommend hammering inside the filler neck with it still attached to the fuel tank. Dirt, metal filings, and the slippery chisel are
not things I want in my fuel tank. Sure wish they made a bigger tank
on our trucks.

Sure wish they made a bigger tank
on our trucks.

They do -> Supertanks and they have plenty!

Ben