I need help with gas filler tube

Your thinking is good. With the first one I did, I think I sawed away at the metal of the clamp in one place, till I could twist it enough with sharp nosed pliers that the metal snapped. I also had a very large screwdriver (17" from Home Depot) to try and pry the clamp around. I didnt use the entire hacksaw, just used a fresh blade by itself. Moving it slowly I figured would mostly eliminate the chance of a spark. And I had sprayed the area with the nozzle on my garden hose.

That was on the clamp closest to the tank. Unfortunately the clamp at the other end of the hose is obscured by the rear subframe. In that case I might have cut through the hose with a good sharp penknife, and work the fuel filler assembly out of the car. Once the large rubber hose has been sliced through, and the clamp for the smaller rubber hose has been remove and the hose worked free, it not hard to get the fuel filler neck out of the car. Mostly have to unbolt that plastic cover, and the two small screws on each side of the gas cap.
On the most recent one I worked on, our 91 hatchback, I also replaced the smaller rubber 'vapor' hose between the metal of the fuel filler assembly and the top of the fuel tank - since it had a chunk that had broken out. I had to unbolt the tank support straps and let the tank sag down a couple of inches at the back, in order to reach the spring clamp on the small hose (which was about 9/16" I think. I didnt have to remove the heat shields or the exhaust system, but did need a pair of needle nosed pliers to get in there.

I expect I will have to do this same repair to each of my Escorts eventually. Not really looking forward to that.