As far as battery supply goes, you are correct that you are in the "specified voltage range." One problem...You will never hit the amperage range that will make this light do what it is intended to do.
Alkaline batteries can not deliver more than about 1 amp, at that point, the voltage of the battery "sags". Alkalines are not high current batteries.
On a positive note, NiMh batteries are able to deliver the current. Rechargeable batteries are rated at 1.2v, or 3.6v when you have 3 of them your Mag 3D, and they will be able to deliver the 2.8 amps required to drive this to 900 lumens.
If you run this off of alkalines, you will see about 200 lumens Max!
It is well worth your efforts to purchase the rechargeablbes and a charger, you wont ever buy batteries for it again, and you will be able to blind your friends from a mile away.
This "drop-in" module is not a perfect match for the Mag 3D. The problem is heat. This LED will put off a ton of heat and the only way of not killing it within the first 20 minutes is to channel the heat through the whole light. On this kind of high heat application, unlike a computer, we don't use fans, or anything to cool.
Your hand will absorb a lot of the heat, but the heatsink must be in contact with the sidewalls of the light.
I recommend searching for 14413 in the search box and looking at how people are doing this in the "3C" sized maglite. It will require slight sanding of the brass heatsink, but it will be snug to the body.
For another $20 for the light, its well worth it, considering that the LED module cost more then that.
The 3C with this module can be regarded as the lowest cost P7 Mag build.
Good Luck.
