I use an Ocean Rhino RX5, which is the longest one they make. Spearguns are measured weird. Sometimes it's overall including extension, sometimes it's tip to trigger mechanism. Anyway, it uses a 60" shaft.
I converted mine to use the Evolution shafts and rings from Speargods. They are a local shop to me, but do quite a bit online, so they'll ship. The evolution shafts make it easy to go from free shaft to line shaft. There are other systems to relatively quickly convert, but then you are essentially shooting a line shaft, with a slide ring and other hardware to attach a line. The Evolution shafts just have a small crimp at the aft which will catch the ring that is attached to the line. You can also backload them, which is a bit quicker, IMO.
One thing, though. Free shafting is not for every location. Check with locals to see if it's done. With higher currents on the east coast, it's often not ideal. If your currents are pretty mild, then you could try it. I took longer than I should have to start free shafting. I was just nervous at first that I would lose a bunch of shafts. Over the years, though, I haven't lost that many. When my daughters started spearfishing, I started them on free shafting right away.