I had a spare 8T laying around so the choice was an easy one.
Here is a email I received from Mike @ RCM after asking the very same question. Hope it helps::

The Neu 1515/1y or 1515/2.5d motor are both excellent choices for the job. I prefer the finned versions myself. Either of these winds will perform excellent on a good 4s Lipo. Speed can be comparable with either motor by properly selecting gearing. Perhaps a 13 tooth for the 1515/1y motor and a 16 or 17 tooth pinion for the 1515/2.5d motor on 4s Lipos.

The best Lipos around are either NeuEnergy, Flightpower or Polyquest extreme series. All of these are very similar in performance and overall capability and I have used them all with excellent results. The Polyquest and Flightpower packs are made from the same cells, so they are of course nearly identical. The NeuEnergy packs are made form shorter and thicker cells(can fit more easily in tight spaces), but are otherwise similar in performance – they do offer more opportunities to adjust front/rear weight bias due to their shorter length.

I think the MGM 224/18 controller is the most capable controller for these trucks – they work best without reverse enabled, but are otherwise considerably over spec’d and can handle the load without need for a fan in all but the most extreme cases. They also have a built in switching BEC, so no external unit or rx pack is required.
::

I went with 1515 1y/f - MGM 22418 - NEU 5000Mah 4s and RCM motor mount.
The conversion was a breeze.