Panel truck gas tank won't fill Update: got it working
 

I went back and pulled the vent tube on the tank, I could blow through it but couldn't get a wire to go through. Apparently it had just enough of a clog to keep it from filling, cleaned it out good and now it works fine. Another one of those things that keep you humble.😁 As title says I can't get my panel to fill past half a tank. After putting in only 6 or 7 gallons it looks like the fuel level is getting above the vent that runs from the top of the tank to the filler neck and it then will only take a little gas at a time. Anyone know a fix for this?

Is it possible your breather is blocked ?
Try blowing some air through it if possible .

The vent line that runs from the top of the tank to the filler neck is there to allow air out of the tank that is replaced with fuel and is normally 1/2" ID hose or larger. Is this a stock newer style tank setup? Need more details on the design, need to have two vents, one for allowing air into the tank to replace the fuel being used while the engine is running and a 2nd vent to allow air out of the tank when being filled.

In this picture of the tank in my F1 the larger vent line on the drivers side runs to the filler neck and the smaller 1/4" vent line runs to the charcoal canister.

Attachment 192210

Only "vent" I know of on a 1950s tank would be the oversize filler neck opening that allows air out while putting gas in, vent for allowing air in while driving would be a vented gas cap. I would say the filler neck is plugged or collapsed somewhere. Regardless of the vent design if air can't get out the gas can't go in.

It's been a while since I've worked on the tank in my panel. I have a non-stock tank that I was able to fit in the stock location but I do have a stock tank on the shelf. The vent is at the top of the tank, is there a tube inside the tank from the fitting at the top that goes down int the tank? I'll have to take a look at the tank I have on the shelf. I'm thinking if there's an internal tube it could be blocked inside the tank allowing gas to only fill to the fill tube.