FWIIW, I have done this job and did learn a few things from the experience. The Ford service manual states to cut away the fuel tank grommet before removing the filler neck.
I normally follow the service manual very closely. When I dropped my fuel tank, I did not COMPLETELY remove the old grommet. When the filler neck was extracted, this broke the anti-rollover check value on the end of the filler neck.
The filler neck check value can't be serviced separately. So a new filler neck was needed. Replacement of an expensive part that could have been avoided.
The gas vapors are hard on rubber parts as it is. The filler neck grommet is a frequent source of leaks. The grommet from Ford is only an $18 part. Replace it and avoid future leaks.
In addition, the re-install will be easier as the filler neck will go back in much easier with a new grommet.
My recommendation. Take the time to completely cut away the old filler neck grommet. Once done, the filler neck will easily come out of the gas tank without breaking anything.
This will give FULL access to the top of the tank allowing for it be be cleaned inside and out.
IMO, the Mustang has one of the easier fuel tanks to drop (should you decide to drop the tank). It really doesn't take that long to do especially if the tank is empty. Just get over the urge to re-use the filler neck grommet and in the end, you will be rewarded with a sucessful outcome.
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.