If you are set on running li-ion cells, I think you will be better off doing one of the following:
1. get a 1 cell extender for the 6P and run 2x17500 size 3.7V protected li-ion cells (AW brand!), use an LED module like the M60F or other similar "buck" regulating LED module, (the M60F will probably be better than any other options because on a bike, you don't want a tight throwy beam like most LED modules are, the flood beam is much more practical)...
2. Get a 1x18650 C-C leefbody or Solarforce L2 body (either available from lighthound, the leef is of higher quality, the Solarforce is cheaper price wise). Run 1x18650 size li-ion cell, (again, 3.7V, protected AW brand!).... See if you can get Malkoff to put together an M30F for you, (it's the low input voltage version of his modules, boost regulated rather than buck regulated), or use another 3.7V only D26 compatible LED drop-in if you don't think you'll mind a tighter beam pattern.
3. Get a 2x18500 or 2x18650 body (from FiveMega or Leef), run the M60F on either 2x18500 or 2x18650 cells. The 2x18500 option will run longer than the 2x17500 option listed in "1" above, and still be backwards compatible with running 3 CR123s as backup, still 9P size flashlight...The 2x18650 will give you the longest runtime, but the size of the flashlight will be like a "12P".... The 2x18650 option will not be compatible with 4xCR123s as backup because the voltage would be too high for the MalkOff M60F.
4. Another possibility, with less runtime than the options listed above but still viable: A single 3.7V 17670 li-ion cells in your stock 6P with a M30F (if you can get one). This requires the least modification and is probably the least expensive option.
The 2x17500 and 1x18650 option will both have about the same runtime. The 1x18650 option will be the most compact, but is NOT backwards compatible with 2xCR123s as the voltage would be too high for the 3.7V M30 module that you would want to run here.
The 17500 size and larger li-ion cells are all pretty decent in capacity for their size, I don't generally recommend any setup that runs on 2xRCR123 size li-ion cells if you can avoid it, as they have pretty poor true capacity. A single 17500 has about double the capacity while only being 1.5X the size of an RCR123.
Eric