I just did the same thing you are going to do about 3weeks ago due to my 16yrs old rusty brakeline leakage right btwn the gas tank & the frame rail. Everything is super easy when your bed is off. (it's worth the effort to remove it).

Since you are replacing those 2 brakelines along the frame rail from the front to the back, just cut your brakelines with a cutter about 1/2" from the fittings. Using a socket/rachet to undo the fitting, you are almost garantee not to strip them etc.

To pop out your old brakelines from the framerail, just use a flathead screwdriver & pry them away from the clip.

At the same time, you might as well clean/paint the area around/on top of the gas tank. You could even tap in a fuel pump kill switch or something at the same time while you have access to it!