Steel spacers would be perfect. I have found that the wheels need to be tightened up super firm, or else, loose wheel... then pin loose and wheel stripped on the rear anyway.
I have picked up some Associated shims for the rear axels. I also noticed that my rear alloy spacers had become squashed deformed after 10 race meets. Cheap to replace though
The car seems to seriously lock down and grip when you fully punch it. I attributed this to some of the wheel bearing being shot after each couple of race meets. I am picking up bearings for about $1 (50 pence) so no big deal. Especially as the gains on dirt tracks are simply awesome. The outer-outer rear wheel bearings on my B4 also fail, but over a longer period of race meets.
Would it be worth buying high quality bearings like Boca if the wheel bearings are taking such a punishing?