Glad you got the chassis and we look forward to seeing photos of it!
I think you'll find that most people use the stock 13T pinion, but there may be a few that bump it to 14-15T. 11 or 12T modified motors with stock gearing makes for a really fast truck.
Make sure that you upgrade your 13T pinion to a steel one made by Traxxas or Robinson Racing though. The stock Tamiya ones are famous for wearing down quickly.