Is there no way you can get at the job from above ? e.g. cutting a
piece of floorboard ? Working beneath the job in a confined space is
certain to be more difficult and prone to accident, e.g. touching hot
pipework / blowtorch, hot solder dripping onto you etc. I've been
under our floor with a similar space recently to run some electric
cables and it can be difficult to breath even without a blowtorch
stealing the oxygen and warming the air around.