Agreed [with your comments] on the kludge of AR-15/7.62 variants. However, since he already has the upper. I might suggest the purchase of another lower [the “AR-47”], that is intended to directly take AK mags,  eliminating the [straight magzine well] problem.
I’ll be honest, these things kinda suck, the fit and finish on them is truly horrible. I’ve assembled a few of them into California-legal configurations, they do work, but getting them to fit with the upper, and everything else is a pain in the a**.
Personally, I would be inclined to just replace the 7.62×39 upper receiver group with either a .223 or 9mm upper, and go that route. If anyone is looking for a cheaper place to pick up those AR-47 lowers, I think the guy with Metroshot  still has some (last time I checked, he wanted about $125 for one of them.) Send him an e-mail if you are interested in one. He’s an FFL , and will ship
to your in-state FFL. Regards, – AVL