There is one re-loading trick that could possibly tighten the pattern and that involves use of buffer to cushion the shot during set-back preventing deformation. However, buffer raises the pressure of the load and I wouldn't advise shooting a buffered load through an old Holland without knowing that the pressure was within limits. The only way to know is to send the loads to Tom Armbrust for testing. It may not make a difference, but if you are going to buy the gun anyway, it would be something to think about doing.
Here are some loads to consider:
https://www.nssa-nsca.org/wp-content/uploads/2011/10/ceretto.pdf