First off I see you are no where near making any decisions. This is all of what you got under there. Before you determine what to do, you have to find out what you are working with. That is bad. But perhaps not terminal. Are any other rails in the same condition? There is no cheap easy fix for rust. Are the rockers sound?

Step one is removing the rust, I sandblasted mine. Depending on how bad it is will make the choice of fix or replace. You can splice in a section of frame rail, it's a huge job. You could sandblast and do a repair from the inside as I have done. I had plenty of good metal to weld to. If you splice, removing the motor and trans would almost have to happen.