I'm not sure you'd be able to fit the bike in a 5x8 enclosed. It looks like the wheelbase is about 64.3 inches, but the total length might be more. It could be a very tight fit and you would have to walk it in.
I would recommend a 6x12 to get this bike secure and safe to haul. A 5x8 is just too small, and possibly not rated to haul a nice Harley like yours.