Ok I found out some answers on this today – I talked with an annalist about this, it’s not in writing so take this info with a grain of salt
You can make an AR or AK type lower as long as it complies with penal code 12275.1(a). However, once identified as a AR or AK and placed on the list of restricted named receivers you will have 90 days to register the lower as an Assault Weapon. The Department of Justice can decide to
Decline your registration of that Assault Weapon, leaving you no other choice than to relinquish, destroy or export that lower out of the state in 90 days. So much for the build it your self idea.
If you can modify the lower in major way so that it can not accept a key component such as a pistol grip, high capacity magazine, collapsible stock or AR upper then that may qualify more than a minor different. If I were you I would have that in writing before you start hacking away at a forging or casting.
1227.6(e) the term "series" includes all other models that are only variations, with minor differences, of those models listed in subdivision (a), regardless of the manufacturer.