Wow, that's very classy looking for a prototype
Very cool project! This car indeed has the weight very far forward - I think the suggestion to flip the motor and use a shorty could work very well - and it seems there's some spare room to move the whole slipper assembly+motor mount further to the back?
With this amount of work on it, it would be a shame to bin the idea (even more so because the Tamiya's part in (2WD) offroading is quite small).
I've got two questions about this project:
1. Do you have a video of this in action?
2. Why did you choose the 502 and not the 501/511?
I would imagine the motor position+rotation direction would help rear bite on acceleration, and the belts will give slightly so the power delivery is smoother (=more grip again). It's the same thing I wondered with the DB1... why a shaft drive instead of a belt drive donor?