My quick thoughts.. It's it doable, yes, is it reliable, ehhh.....
Basically the reason companies struggle to make a reliable 7.62x39 AR15 is because of the shape of the casing. Looking at x39 vs 223 you will notice the large amount of taper to the x39 round. This requires a magazine that is curved... (ak 30 round "banana" magazine). Now look at 223. Straight casing gets a almost straight 30 round Pmag.
Now mix the two.... the tapered x39 round has trouble feeding through a straight magwell, and needs special attention to the feed ramps in the upper. If not expect failures. Also x39 specific AR15 magazines are not overly reliable. Thus enhancing your problem.
My thoughts are yes do it. But do it using a MGI Hydra lower. This lower has easy change magwells. And you can swap to a magwell that accepts all AK47 steel mags.
Or buy the upper now, run it on your stag, if you think it's unreliable, build the HYDRA lower. And you'll have 2 guns.