1999 Buick Century Replacing Gas tank

When the gas tank is over half full, the gas fumes are really bad. I think the gas tank needs to be replaced.
I asked a mechanic I know about it and he told me it could be very involved and costly. I wanted to get your opinion on how involved and costly it would be to replace the gas tank. The car has been well cared for. Upstate NY winters are tough on the cars from the salt. I think the tank may be rusted.
I can only put 6 to 7 gallons in it at a time because the fumes are horrible if I put more than that.