I have a 5'7". Things like lumber aren't really an issue with the tailgate down; but haven't hauled anything over 8ft. The 6'4" would be nice but another 9 inches is an issue for my garage. Not to mention that I have a grille guard on my truck so I can't move it forward in the garage without a lot of re-organizing.
If I had a quad cab instead of a crew cab, then I would go with a 6'4". As for the ramboxes, I like the idea but I've read a number of negative comments on this site about about the quality of the fit/alignment. Doesn't seem to me that any golf cart I've ever been familiar with would fit in a rambox; think you would need a trailer.