First of all, what year are you working with? 1947/1953 used the same sending unit, 1954/55 used a different sending unit with a fill tube, the earlier tanks filled from the middle. Do you know which sending unit is being used?
But I also noticed that you put por-15 in the tank. Did you remove the tank and rotate it to make sure all of the tank got sealed?
Here is a previous
POST that should help with understanding the gas tanks on the panels and suburban.
