Changing any of the rubber lines can be tricky, the special copper washers have to be very tight. For the frame connection they usually never leak, unless the fitting is damaged, not allowing it to be tightened enough. The proper flaring tool is very important, those fittings need to be perfect.
Sometimes they will leak even when you think that they are more than tight enough. It's best to inspect those washers very carefully before installing them. Use the very best fitting socket for them, and tighten them carefully but firmly.
For your truck you should attempt to use the proper flare nut wrench to tighten that connection. Take your time with it, worry most about the frame line and fitting, it's permanent.