The first point is the meaning of SLM. The "standard" temperature in SLM should be 0 degC according to this and many other sources; https://en.wikipedia.org/wiki/Standard_litre_per_minute Particularly this Teledyne Hastings page mentioned "Teledyne Hastings Instruments assumes STP of 0 °C and 760 Torr"; https://info.teledyne-hi.com/blog/topic/standard-temperature-and-pressure
Then about the volume conversion, I cannot find any handy webpage written in English. But using this NIST Chemistry WebBook: https://webbook.nist.gov/chemistry/fluid/ The density of gHe is 22.426 L/mol at 0 degC and 760 Torr, which is almost the ideal gas. On the other hand, LHe: 1 L --> 125 g --> 125/4 = 31.25 mol. Thus the volume of gHe made from 1 L of LHe is 31.25 mol * 22.426 L/mol = 700.8 L (at 0 degC and 760 Torr).