Bigglesbro, it was not just g-pants that he would have been wearing but a full partial pressure jerkin with sleeves and a Taylor partial pressure helmet. The latter allowed FL660 rather than the FL560 for the P/Q mask. I think the sleeves on the pressure jerkin permitted even greater heights to be flown. These were of course oxygen equipment limits and not necessarily air frame limits. IIRC the Lightning had the Mk 21 regulator that could deliver 30mm overpressure at 45,000ft and 70mm at 56,000ft.
Later our Mk 21s were downgraded to the Mk 17 which meant we could go to 50,000ft with only the P/Q mask. Then someone discovered aerodynamic suck which reduced the permitted oxygen flight levels by about 2000 feet.