how olds the tank? if its more than 6 years old your probably better
off just replacing the entire tank, since the gas valves are pricey...
be prepared to replace the tank drain valve, many are plastic or so
cheap if you get them open they will never close again....
open the T&P valve to let air in so the tank can drain, be prepared to
replace that valve too, once disturbed they often dont shut good and
have a piece of pipe to put on the end of the wrench to remove the
thermostat it may be near impossible to get lose.
short and long start first thing in AM on a day part stores are open,
since this job may expand repeatedly:(
If your lucky it might be a snap