Back to the original topic, what you need is more thrust. Plenty of spare power will get you nice and high. Handley Page's finest bomber, the Victor could get to FL600 with no problems, I flew one of the Tanker versions with it's shorter span up to FL520, and we could cruise at M.9 at FL470. There was nothing to match us in those days, but you can find some GIVs and Citation Xs up there now.