Lens' photos show the only issue with building a 5x8 on a 4x8 frame, the fenders. However, the fenders are easily compensated for.
If you have the time, definitely go for the 5x8. My new build is a 4x8 simply (and only) because I need to build it very quickly and lots of things come in the 4x8 dimension. It is a timesaver only. Good luck on the build!