I'd just keep my eyes open and watch for a 3 inch barreled gun. they show up from time to time and it will most likely be far cheaper than modifying an existing gun. 3 inch Smiths arent' THAT uncommon.
It took me several years to find, but I LOVE my 3 inch barreled Detective Special.
Mark Lane to William Buckley: "Have you ever referred to Jessee Jackson as an ignoramus?"
Buckley: "If I didn't, I should have"