Clive,
Rather than just fitting the standard size spacers, I would remove the chain and lay a string along the side of the sprockets to check that both sprockets are correctly lined up, and then fit suitable spacers. Remember, these bikes have now had more than 60 years of, maybe, not so careful use - so the frame alignment may not be as it was when it left the factory!
The harsh feeling could be caused by a problem with the drive shock absorber - take out the spring and reassemble to check whether the cam gets close to riding over the top (there are 2 lengths of splined sleeve available and you could have a mismatch). If it does, add a 1/16" spacer over the splined sleeve to restrict the travel, regrease the shock absorber and assemble. Make sure the through bolt is fully tightened (the whole assembly should be clamped solid with just the sprocket and cam able to move on the splined sleeve) and splitpin replaced. Roadtest to see if there is any improvement.
Regards,
Eddie.