As Bob says, the trailing arm for the 6 is identical to the 4. Post a "Wanted to Buy" ad on the
914club website and I would bet you even money someone within the area or OC would have one for you fairly cheap.
If you don't have the stub axles, then that's a different story all together. Original 6 stubs are going for $600+ a set (if you can find them at all. They are quite rare) or you can get non-original knock-offs on the Pelican Parts website for about $450. Other knock-offs may be out there, but I haven't shopped around.
Good luck!