Old cranks seemed to be much more prone to splaying than the new cranks. I've used brass shim stock with success but I suspect you could cut up a beer can and try that for shim stock. It's usually a pain to keep the shim stock on the taper when your fitting the crank arm so just glue the shim stock on with some super glue.
Jim Cushing-murray
Where the Surf meets the Turf