What crank is it, I believe they were all cast iron other than the Inter and Manx so welding a "standard" crank might not be a good idea.
Another good trick is to drill and fit a small roller, Half in the flywheel and half in the main shaft, So the shaft can't turn. Cheers Bill.
.004 is as good as you can get without galling. If your flywheels turn to butter at anything less than .003, I'd buy a proper hardened flywheels from Maughans.Try for .004". You'll need a good press to do it.