To update the info on my issue, here is what I found, and the solution. I dropped the gas tank a few inches to see what was going on with the breather hose. It was very clear what the problem was, as the hose was kinked in at least three 90 degree bends. There was no way that the tank could be vented properly to allow it to be filled. I understand what went wrong with the installation, as the hose needed to be pulled from the top as the tank was lifted back into place in order to keep it from kinking. Once the tank was back in place, I filled the tank with no trouble at all. In fact, the gas gauge now also works as it should. If anyone else has had a similar problem with filling their tank or overflows, this seems like a reasonably easy fix.