Okay, quick update...
Got the AE87 and at first it looked the same width as the Tamiya but indeed it is fractions wider (why did I doubt you Jimmy?!?).
The extra width, plus some "ecomony" with the motor shaft depth into the pinion has given me (apparently) sufficient spur/pinion overlap. Fingers crossed.
I will try the ratio suggestions from Fredrik as I now can use my 21 - 23T pinions...
Will see how ~8 goes instead of 7.0.
@ 4wdfull - Not road tested yet but my X12 motor, Robinson Racing pinion, AE87 spur combination leaves plenty of clearance for the belts - the pinion and belts do
not overlap in my install.
Pics of my build to follow in a new thread... am quite pleased with it actually - a gem compared to the 501X.