Floor Bored under body question?

i have a 1990 Dodge Omni and when i was puting a new Gas on it this week i was looking at the undbody of the car and it looked like the drive side floor bored under body was starting to give out how can i fix this the whole under body has rust from front to back i want to keep the car cus the motor and tans and the body is good please kelp ASAP