Hi Adam,
drifted one wheelset out to EM back to back, and can just see the splines on the axle ends!! So thats that answer for you!
Had thought of u shaped plasticard.....if they can be successfuly glued to the chassis plastic. Think thats a better option than warehouses of Peco washers, thanks PMP, but its something approaching 2.5 to 3mm play each side to take up!
Am still tempted to go for wheels off and brass or biro tube spacers!
By the way, very impressive buffer beams Bubbles, nice work.