In the old days we used to shim the spiders tight in a 9" and it would perform as a posi for a while. But, in reality the carrier has to be made for the clutch pack that fits inside. This pack provides the friction to turn both wheels. A non-posi carrier is different from a posi. There is a gentleman in Ohio that specializes in 8" Ford posi units. Maybe someone here can post his name/business as I can't remember it at this time (Geezeritis).
Ford calls their units Trac-Lok (short for Traction Lok), Posi is the inferior. Both 9" and 8" are Trac-Loks. The rears were available in 2 & 4 pinion "open" diffs (non Trac Lok) and Trac-loks.
Both have some parts than interchange. 8" Trac-loks are hard to find, there were alot more 9" T-Ls made. You can shim the side gears in an open rear to tighten it up and have it act more like a locker.