Yes, I have some recommendations, but the problem is the availability of the parts.
In the first place: you have a SIX D! Congrats!! All you have to do to get the widely used rock solid 7.2V is to put six high quality NiMH D-cells in it instead of six Alkaleaks. That's step number one.
Step #2: replace both the reflector for a completely metal one, as well as the lens for a nice glass one. Fivemega has both parts available, cammed reflectors in smooth finish is what he has left. I can only recommend you to buy these while his stocks last, these are one of the very best available. ALL Fivemega stuff is gorgeous!
Step #3: now that wee bulb has to make place for a powerful one. If you can still get them (Tad customs used to have them, but shut down is site, and now we have to wait until he open his ebay-shop) look for a PR to G4 bi-pin adapter. If you finally managed to grab one, you only have to buy a WA1274 from Fivemega and let the fun begin!
Alternatively, Fivemega has his super bulbs. Potted / frosted Carley bulbs 7.2V / 1.9A and more nice potted bulbs, like the ROP lo and high in 7.2V. These ROP bulbs don't even require a bi-pin adapter and can be swapped the easy way. They are the same model as the original Maglite bulb.
My go-to setup for a nice bright maglite is just this! I prefer either the WA1274 or the 7.2V / 2.5A bi-pin G4 bulbs from Tad Customs. In this particular case, his PR to bi-pin adapters are better suited than those from Fivemega, because the bulbs are sitting lower, resulting in a better focus.
In any case, have a lot of fun modding your 6D, it is one of the best Mag hosts available, due to the easy way of obtaining 7.2V for the powerful bulbs. Add 6 HIGH QUALITY / high capacity NiMH D cells and half the work is done already. Due to the high current consumption of these bulbs, your batteries have to be as good as possible. I recommend Ansmann 10Ah D cells.
There's been lots of threads over time on this unit, but I wondered if anyone has recommendations for the best you can do on lumens by converting a 6D. Does anyone have recommendations?