If you haven't already purchased the new sending unit consider going with a reed style unit. Remove the old sender and measure the depth for the new unit.
Also, although the screw/bolt pattern looks like you could rotate it and the holes would still line up but it wont. It only goes on ONE way...
I forget the exact replacement size but for me with 4 scuppers as you probably have It made more sense to switch to ball scuppers. I also changed the brass tubes for good measure.
Tap it with a soft blow hammer and see if it moves in. Look inside to see if any hardware is missing or loose.
Sometimes It's just the way they are. When it's assembled it's not an exact science when they trim off the excess.