As I recall, I think I have a thick and a thin Tamiya one on the one-way and none on the bevel gear. You can't really move the bevel gear in too much or it will bind bad.
The LW shaft or the later TB-02R shaft is a little longer and they hold the bevel a little "steadier". It's kinda hard to explain, but the shaft is a BIG part of the system, since there's only one bearing on each end...other shaft cars have two in each gearbox.
I wouldn't even put any shims on the bevel. The only way you can really know if it needed them there is to partially assemble the car (both gearboxes, upper deck, spur assembly, motor mount) and check to see if it's binding...add shims until it binds, then remove one. Mine's perfect w/o shims on the bevel gear.
I think I have a couple of one-way gears left...you can't get them locally? I usually get mine from Speedtechrc.com or at SoCal. If you're in a bind, I'm sure I can send you some, but you should be able to get them...PM me if you need some...