I think you are on the right track in that the fill passageway into the flower pot is plugged up with crud. If it's plugged, I suspect there is enough debris in the tank that you will have to remove it for a thorough cleaning, so I'd just dig right in and do that. I had exactly the same problem with mine, and after a series of flushing with water, sloshing around, emptying, repeat, I got it pretty clean. Then I dropped a length of chain into the tank and rattled that around for a while to scrape loose as much scale as possible.
But then I just put the tank back in the car and started using it. This is obviously not a long term solution, but I figure it should be good for a season. My goal has been to get an immobile car operational enough to fully assess it's condition and come up with a good long-term restoration plan. I figure the prescription for the fuel tank will be either lining or replacement depending on the level of restoration I decide to do.
As you read through the posts on this topic, you will find many people recommending that the tank be sent out to a radiator shop to be professionally boiled clear, etc. It's nearly impossible to do that in the US now, and the shops that will do it actually send the tanks to Canada for treatment, so there is a chance you might still be able to get it done where you are.
It also sounds like lining is tricky. First, you need to be sure the lining material doesn't react e.g.melt the plastic flower pot. That would be really bad. Second, you need to be sure that you don't plug up the various apertures and lines into the tank, including the fill passage into the flower pot you have plugged up now. There are post of people doing it successfully, but it sounded tricky enough that I opted to ignore the problem a little longer and put my cleaned tank back in the car.
