The shrinkage will likely be a problem in that the lead will be loose so it will not do a lot to dampen chatter.
However, I think a potential problem is the heat. If the castings were not stress relieved before the machine was made you may finish up stressing releiving when the molten lead is poured and and that could be bad news.
I know the melting point of lead is actually below the temperature for stress relieving but you would have to overheat the lead to make sure it filled all the crevices; in fact you almost have to get the castings up to the lead melting temperature which means part of the cast where the hot lead first contacts are probably going to be well above.
An open mind is a virtue...so long as all the common sense has not leaked out.