Allicia,

Unfortunately, you cannot substitute plums in the recipe for the peach sauce in the Ball recipe.  I have looked for a  research tested recipe and have not found one.  I would recommend canning your whole or half plums and then making it into a plum sauce once you open the can.  We always recommend following a research tested recipe when canning to ensure a safe final product.  I have attached our resources for finding research tested recipes.  Please let me know if you have questions.  

Kellie Jordan

Food Safety Instructor - Think Food Safety

Michigan State University Extension

Calhoun County Extension Office

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