Welding oxygen is just as pure as medical oxygen. the only possible difference is the purging of the tanks. They both come from the same sources.If you're worried about contaminants from a welding oxygen tank, a filter van be put in the oxygen line after the regulator. These are usually available from the same welding oxygen suppliers.
The numbers you're quoting on your regulator refer to the pressure in the oxygen tank or how full the tank is. They do not refer to the flow rates of oxygen from the regulator. What settings are there on the regulator in terms of flow rates? eg. a pediatric regulator used for most ozone therapies go from 1/32 lpm (litres per minute) to 3 lpm in incremental settings. Most regulators have variable settings. The only difference between regulators is the ability to control flow rates. Regulators meant for breathing start at 1 lpm and go up to sometimes 5 lpm.
A 640 litre tank is still a small tank by oxygen standards but at 1/32 lpm flow rate will give many hours of oxygen.
hope this helps