On my website you can see a couple of examples of crevice corrosion of propshafts caused by poor fit. Putting the incorrect size prop on the shaft might well lead to similar problems. http://coxengineering.sharepoint.com/Pages/Crevice.aspx
On my little lathe, machining the shaft would be easier than the prop, as the set up is straight forward. If desperate, I would fit the prop with thickened epoxy to even out the tapers.
DW