Don't worry about the little white vent in the top of the tank. None of the roll-over valve/top tank vent designs from the factory are going to help with filling. They are just too small. Their only purpose is to let air in as the engine is using fuel out of the tank.
Like someone mentioned in a previous post, you should have a hose within a hose. With all that fuel going in from the fuel station nozzle, a lot of air has to get out at the same time. All this air as to be routed around the fuel coming in, or it will interfere and make the fuel splash and gulp and make the nozzle cut off.
Gas and diesel are the same. The earlier trucks (around 86-dwn) put the fuel into the large hose, with a smaller corrugated plastic hose in the middle that directed the air out away from the fuel going in. This was not the greatest design, and they changed it later to the fuel going down the smaller center rubber hose, and the air going up the large hose.
Since someone took your hose out of the center, you have a pretty bad situation, though even with the factory setup it was not that great. I had the same problems and tried many different things, what finally fixed mine for good was what someone mentioned previously, you take out the white roll-over valve in the top of the tank, and replace with a plumbing elbow that fits the grommet snug. I found a 3/4 copper L fits perfectly, and I soldered a short piece of copper to it, and then got some hose and clamped to that and ran over to the filler area. What I did was just run the hose up inbetween the inner and outer body cavity and tied it up with a screen over the end to keep the bugs out. It works great this way. If I let it fill to the top, sometimes I will get a little spot of diesel come out of the vent pipe at the station, but it's not much at all.
I would not worry about your fill pipe, I don't have the center one in mine now either, and it does fine with the larger vent hose coming out of the top of the tank.