The 40 CF wont give you that much in the way of "fill" power... Maybe twice before you have to refill it... BUT.. Go to a welding gas shop, and have them either sell you the parts, or assemble the parts to allow you to transfill. Essentially 2 CGA-540 fittings on either end of a high pressure hose. Things that make it BETTER are a small bleed valve in the hose. Closed when filling, then when both tanks are closed, you bleed off the pressure in the line before loosening. Not necessary but goes easier on the threads than loosening a hose under pressure. CGA 540 valve stems need to be on both tanks. This is the Compressed Gas Association's industry standard.. and this way these tanks and your hoses will be compatible with other oxygen filling systems elsewhere. The CGA 540 specs out thread pitch, fitting diameter and fitting length, to prevent incompatible gas systems from being connected. Different gasses have different CGA numbers. The guys at the gas shop should know this. It would be a VERY GOOD IDEA to have the steel tank inspected first, and ensure that its within hydro (hydrostatic test date), especially if its been empty. Again, gas shop can help.