anyone know if my model "M" will fit in that stock?
No, the Fab Defense chassis requires the standard pattern of SKS trigger and magazine well assemblies, as it pins these in place and replaces the magazine release.
However, the Archangel SKS chassis looks very similar, and should work flawlessly on a model M. Its system adds a mag release onto the standard archangel SKS magazines themselves. So if a regular SKS stock will fit a model M, so should the Archangel.
*I've also heard the Matador Arms chassis is pretty good, but its heavy and very pricey (~$500).
Hope this helps