I just finished my (rather simple) brushless conversion of the Cen GST. Here's what I used and a few close-up pics of how it looks all hooked-up. She runs great, but I needed to dial in a lot of negative expo (-25%) to get her to accellerate smoothly.

1 Motor - Castle Neu 1520 1600kv
2 ESC - Mamba Monster V3. Space beside motor too small and had to mount the ESC to the carry handle.
3 Motor mount - Tekno RC for Castle/Neu motors
4 Pinions - Stock 17/21t clutchbell pinions threaded onto RC-Monster 2-speed Pinion adapter for 5mm shaft
5 Spur/Gearbox - Cen GST 7.7 stock spurs and 3-speed gearbox (no mods)
6 Batt trays - Tekno RC batt trays with 10mm foam tape underneath and fixed to TVP's with velcro Tekno RC batt straps
7 Batts - 2x 5000mah 5S 18,5V 30C lipos in parrallel for 10000mah 5S 18,5V output.
8 Removed 5-cell Rx hump-pack. All servos driven by main batts through Rx.
9 Mechanical braking done by throttle servo via a servo Y-lead. No braking and no reverse programmed into ESC. Because I used a Futaba 3PM system with the Castle ESC, I had to reverse the throttle channel. This resulted in the brake servo applying brakes while holding full throttle. Therefore I had to reverse the servo direction by switching the servo Potentiometer- AND Motor wires and re-soldering them.

This is a quick and simple conversion. If you want to eliminate the gearbox in favour of a single spur/diff set-up, it will become much more complicated. I am too lazy for complicated. This set-up works and she is plenty fast! It also has the added advantage that it can be switched back to nitro quickly and easily.