yeah, i have looked at the HD too. they sell a copper pipe adapter that you can braze to copper tubing that gives you a 1/2 inch female pipe thread. then you could screw in a 1/2 in brass fitting with a 5/8 barb so that you could connect your hoses. but now your talking about a LOT of potential leak sources.
my other thought is to use an old set of needle nose vise grips and put a crude bead roll on the end of the copper tubing. seams like the best solution so far. other than just using the stock 5.0 heater tubes anyways.