Uh, how many ARs are there in 7.62x39? I think Del-Ton sells a kit for it, or used to, and Colt made one once upon a time but doesn't anymore. Google tells me Double Star makes one in that caliber. Olympic probably still makes an upper in that caliber. None of them seemed to solve the 'needs good mags' problem, though, and ARs tend not to like dirty Commie ammo as much. Let us know if you find anything, though!
ARs in .308 Win are becoming a little more popular in terms of who makes them (though something is still lost in translation and they aren't quite as reliable as their .223 Rem and 5.56x45 counterparts). Good mags exist for these, at least, and soon to be better ones once Magpul releases their polymer .308 mags (>$20 apiece, eat your heart out SR-25 mag hoarders!).
What do you want the rifle for (what do you plan to do with it, and what do you expect from it)?