I think its a great idea, as its next to impossible to buy a new 7.62x39 upper or AR mags in 7.62. Its not round that is ballisticly inferior, its usually the rifle spitting it out. AK's, the leading spitter of 7.62x39, are not precision rifles. Therefore you do not get a tack driver, and the market responded with cheap ammo, perfect for non-tack-driving rifles. My Russian SKS can shoot 3-4MOA out to 150yds with cheap russian ammo. But you can buy good ammo or load it yourself. BTW, many people hunt with 7.62x39 and find it quite effective against deer and other medium game.
On the other hand, I seriously doubt that Atlantic firearms will ship you the lower alone. To make it Kali legal, you could put a set screw into the mag release paddle to prevent it from moving. Along with a 10 round mag, once the mag is fixed you could bring it into the state. you need to find a person to do this, and that person would need to have a 01 FFL (otherwise could be seen as straw purchase).