I am not sure I would rely on an internet recommendation to cut a bulkhead.
Sure your boat was a factory built boat. One of many... Are you the only owner? The boat is 20 years plus. A Previous Owner might have decided the factory did not know what they were doing and modified the tank. May even be why it is leaking.
First empty the tank, wash the tank and hoses out, then empty the tank again. What ever was in there will come out so make it as little and as sweet smelling as possible.
I would get a flashlight. Remove everything that is loose from the area you will be working. Put it in another area of the boat or on the dock.
Get a helpful friend...
Inspect the tank by opening all of the accesses to the tank and see if it comes out without cutting anything.
If you decide cutting is a better choice than unscrewing the furniture, then cut once and enough to remove the tank as well, enough to put the new tank into the space.
I had to cut my bulkhead I tried to leave enough for the bulkhead to maintain it's strength, yet allow me to remove an engine.