Get the reflector+driver+heatsink... you will spend a little more, and you will save a lot of time.
But keep in mind that the reflector+drop in module is not enough to heatsink a P7, so you are going to need to machine a piece to make the "path" from the brass to the main body, and of course that is in case it is made from a metallic material.
Another opction would be to get that light, change the o-rings, use tons of PU, and a polycarbonate lens to make it waterproof. Then you remove the tailcap, cut the body, and fill it with PU or Epoxy after soldering the wires from the canister.
And if you don't mind to, I suggest you to change that NiCd pack for a Li-ION pack...