1) The AR-15 bolthead must be hogged out to take the base of the 7.62x39 case. This leaves the bolt head weak.
2) There have been numerous attempts to make the 7.62x39 round feed in AR style mags. I don't think any of them are fully successful. What you need is an AR lower modified to take AK mags.
The CMMG mutant uses a modfied AR-10 upper with an AR-15 lower modded for AK mags, but it is pricey.
CMMG MK47 AKM Mutant 7.62x39mm Semi-Automatic Rifle