In the off chance that you have a throughull wrench and can get access to the lock nuts with a decent wrench... and can borrow an extra set of hands to keep things spinning around outside if it isn't bedded in 5200... and you absolutely must keep the pieces in good shape, go that route
I normally work alone, and consider the old tube and sender are worthless...
I use a 4 1/2 inch grinder with 36 grit grinding disk on it... Grind off the head
of the old plastic mushroom then see what you've got for sealant
. Stop when the plastic starts to curl up, and peel the 1/8th inch ring of plastic off with a pair of pliers after scoring it with a knife.
A sawzall blade and a cutting it into 4 pieces with a light touch, followed by a pocket knife normally has the old tube out in about as long as it took to type this, 5200 or not... A 1/2 inch wide chisel and a few pops cut the sealant
and pry the plastic away from the hull.
Once its free from the hole a few smacks with a wooden dowel and a hammer normally unseats the inside locknut, even if 5200'd into place.
If it is bronze, you can do the same thing... By the time the mushroom end is ground flat through the heat has softened the sealant. While it is hot, hammer it through to the inside of the hull. 3-4 minutes grinding and a toasty red ring of bronze falls on the ground...
If the hole needs to be made bigger, you can either use a wooden plug
jammed in the hole to center the holesaw, or use an Ooops adapter that has threads for two hole saws. Center the original size, and hole saw it to the right size. Start it backwards until you are through the gelcoat
so you don't get any tearout.