10-12 years ago before the 3X30 ACOGs specific to 7.62x39 came out, we used .308 3X30 ACOGs on our AK's. I put an Ultimak on my Polytech and it worked fine, took a little getting used to initially, but it worked well.
Later we got Arsenals and they had the side mount, so we switched to side mounts and after Trijicon started making the X39 3X30 ACOG, I moved the .308 units to the M1A's and bought the X39's for the Arsenals.
In short, they will work either way. I prefer the side mount rails. The drops are more visible to my eyes there. 300 was the best I got with the old setup with the Polytech and the .308 ACOG upfront. Now it's nothing to get 621 out of the X39 ACOG and the Arsenals. My wife's, my son's and two of mine are set up that way and they all hit at that distance if the shooter does their part.