Bamaboy has you in the right direction. My BIL also adds 10# of dry ice for every 100 gallons of tank. The CO2 will push any residual gases out. Keep in mind propane is heavier then O2 and will collect in lower areas. Cutoff wheel is the way to go. The ones I have seen cut they will draw out the line to cut, then score the line with the wheel and repeat process. They say it saves the blade and you get a cleaner cut.