Thanks for all of the above, but some of the comments assume that I would be able to confirm the base of a FL55 airway is not active etc; and that would mean contact with ATC/ATSU. This is Class G airspace, with no requirement to even switch the radio on, and if, on a nice day, (below 1013hps), I want to demonstrate spinning, I had assumed that up to 5450' would be available, without conflict, and only needing a very good lookout!