The 3" M13 barrel would require some work to have it fit up correctly--most likely the rib would have to be machined to match the narrower M15 rib, and the fixed sight milled off and replaced with a dovetail sight or with a a sight base allowing for interchangeable blades, something akin to the old Weigand system.
2" M15's are pretty sweet. I like mine; I'd wanted one for years ever since reading about them in the fourth edition of "The Book of Combat Handgunnery," by Chuck Taylor:
And they are accurate: