Be careful. The brake cams on BSA`s will impart a rubbing action, direct on your new "shims" and wear or displace very quickly if using tin cans as your shim source. If it jams it could pitch you off or at least damage the drum further. The other thing that may be possible is to reverse the lever on its shaft as they usually are made offset to the square and may position your lever at a more advantageous angle. Alternatively, get some 5mm stainless sheet and custom make your own lever - if you have access to a vice, drill, hacksaw and square file - or if your lever has a square hole, make it an "8 pointer". Much easier than you might imagine.