Not sure if it'll help, and my memory is foggy, but I'll give it a go.
At some point when I had a Buc with the PVC tube I changed over to a Harken small boat furler and a jib with the luff wire. I had a short wire pennant made up, maybe 6 or 8 inches long, that I placed between the mast and the swivel. Also had the wire under the deck sized so the base of the furler would be at deck level when tensioned by the under deck wire. Since all this was not the original setup, all lengths were more or less custom and accomplished with my best guesswork. To complicate things I was also using a jib from a different model of boat.
Maybe your boat was also modified by a previous owner at some point. But you should be able to setup some type of pennant/swivel combo to size it to the jib. Not sure if a foot and a half is what is typical, or if you plan on racing one-design, but you might want to check the Buc website for specifics.