Nighthawk45, that Amerigun USA would be great, except that scrolling to Sig P220, and that operation, it showed an "NA" by the cost, which I believe means "Not Available".
While it maybe possible to dovetail the front, I would use a knowledgeable gunsmith for the operation, as it is a folded carbon steel slide, and not a milled forging.
A lot of the value of your gun, is the slide itself, of course the matching serial numbers help, but the Herndon address, and that fixed front is part of it's character.
Another course of action, is possibly getting a Caliber X-Change kit, with a whole new slide assembly to attach to your lower, preserving the original slide assembly for posterity!