1997 Buick Park Avenue rear break lines

I was told that the rear break lines on this car are completely rusted and need changed. I would like to do the job myself. I am told that the job is not to hard except for one thing. I heard it is very hard to get the new line up and over the gas tank. I'm told that there are two separate lines going to the rear breaks, one for each wheel? How difficult is it to change these lines and is there a trick in order to get the lines up and over the gas tank. Will I need to lower the gas tank to do this job?