What's up is their a down side to opening up the fuel nozzle opening to take the bigger truck size pump nozzle?

Model year?

Your fuel tank vent is the outside area of your filler pipe. The fuel goes down a seperate pipe in the center of your filler. If you remove the restriction you also remove the vent and the fuel will come out at you as the air is escaping the tank. Buy a "Transfer flow fill neck kit" and your problem will be solved. Been there done that. Works good. Only takes about an hour to install.

Your fuel tank vent is the outside area of your filler pipe. The fuel goes down a seperate pipe in the center of your filler. If you remove the restriction you also remove the vent and the fuel will come out at you as the air is escaping the tank. Buy a "Transfer flow fill neck kit" and your problem will be solved. Been there done that. Works good. Only takes about an hour to install.

how much and where do you get them?

I can only speak from my personal experience w/an 02. Upon acquiring the transflow kit and removing the oem, the opening was the same size ~ it did change the venting method tho. I never had an issue w/nozzle size altho many reported they do/did. OEM has a plastic pipe that goes to the bottom of the tank and the space between it and the fill neck plumbing serves as a vent. Very poor design. Still w/the transflow, fill rate might be a lil faster ~ still can puke it if not careful ~ so in my case (26Gal tank) its questionable the expense is offset by benefits. Again, mine is a late/one of the last 02s. Your results could vary.

It does not work to just take out the pressed in vent....I think I still smell like diesel fuel. I got the Transfer Flow...it works great. The new '05s have the better filler already installed....sucks for us LB7 drivers.