I believe the voltages will give you less battery life than you expect.
2xCR123 head: 3-9v
So as long as your 18650 stays above 3v you'll be happy. I have no experience with them so I don't know how long you'll stay in regulation, but it will certainly drop out of regulation before the protection on the battery cuts in - A feature to some, a fatal flaw to others.
Get someone to confirm this, but the .9-4.2v head may be better for the 18650 voltage range. You'll trip the protection without warning, but voltage rebound and lower modes will cut you a little bit of slack.