@pullstarter: Thanks. Sorry I didn't see your reply sooner - it must have slipped by. As to what motor to use; I don't know. I'm sure any S can motor will work just fine if geared appropriately for the voltage applied to get at least 30-35mph.
@GO-RIDE.com: Welcome to the forums! I'm not familiar with the GTB systems, so I can't really offer advice. I was running 16T pinion and was getting some overheating using a MM and 4600 motor on 2s. I have 12T, 13T, and 14T hardened pinions coming (hopefully today) so I should be able to find the sweet spot with one of those. I'm thinking the 13T will be the ticket.
@cemetary: No updates yet. Like I said above, I have some pinions coming. I'm hoping to be up and running for the Wed night electric-only races (that is if there is a class for the CRT.5 to fit in).
Both the 4 and 5Ah batteries are very close in size. If you aren't sure, just make a tray that will fit both. :) There is room front-to-back to fit either of those. I'm using 2s 5Ah packs, so they do fit. I also made the tray usable with 6 NiMH cells just for kicks (pretty nifty if I do say so myself :)). The pic of measurements below was posted a while back, but here it is again for your convenience:
