Can Back used to make soft tops for pickups, but good luck finding one. For a K5 top, how would you close off the front of the topper? Attach it to the cab? Maybe you could have a boat shop cut the front of the fabric off and sew in an extension. This would probably only work with a snap top (add a couple of snaps in the extension) and not one of the fast-trac ones.
What is the difference in length?
But why do this? Save the pricey K5 soft-top for a K5 and build your own soft-top. Does it even need to fold down? Just build a frame, bend some hoops and have a canopy or boat shop do the fabric. Probably cheaper.