Hi Huddie
Did you have the timing side bush replaced ? I can't recall.
If so it could be the new bush has a bit thicker face - just make sure it is seated right down into the case, careful here use heat on the case and pull the bush in rather that the tempting "give it a tap" method.
Don't know much about the balancing procedure, do they split the crank shouldn't think they need to but others might like to comment here, just wonder if the crank could "grow" in those circumstances.
I'm only a two As guy so can't say what the norm is but my last one had very little clearance, it did need a shim and ended up just on 1 thou so quite possible all is ok, no reason to think BSA couldn't work to close tolerance even all those years ago, if once bolted up it spins free with the old bearing and the new one is no more than 7 thou more I'd be happy, once again there are members who've done this more times than I and no doubt will give more info
edit
OOPS 7 thou more, should have been a decimal point and a couple of zeros in there
sorry