Rick Ehrenberg also sells them on the 'Bay. Search item 225500143350. He's a little cheaper than Mancini, with free shipping to boot. Your current power coupler will work with the adapter/extension. Its a 1-stop solution for both length and spline.
These extensions were originally factory parts, used on B-vans during the 1980s. The OE part number was 4037651 but it's long discontinued. While you're working with it, consider rebuilding the coupler (it makes a huge difference). Visit/call your local parts store of choice and ask 'em to order you a Dorman 425-253. That's a coupler rebuild kit that'll set you back around $30 or so. The last good Mopar number for the rebuild kit was 4443436AC, superseded from 2800738. Alas, it too is no more.
