Align it the same way you bore-sight a scope. Just because they're irons doesn't mean you can't do it!
Put the AK barrel assembly in a steady rest. From the breach, sight down the bore at an object 25-50 yards away, and lock down the steady rest.
Adjust the FSB until the sight picture is centered on the same object as seen through the bore. There's your centerline. Make a witness mark on barrel and FSB (paint dots would do), and you're ready to assemble.
Same thing can be done on a built AK, but you may need a mirror to sight down the bore after stripping out the receiver internals.
I have two rods that are about 12" long. I set one on the rear sight base (centered) with the sight leaf removed. The other I set (centered) on the front sight base (through the machined opening in it).
I then can sight from the rear of the rifle, and tell if both rods are in line.
For a visual it's something like this:
(borrowed picture from >>>Here<<< but same idea)
When I replaced the barrel on my Yugo M70 with a new one I lined up like this with great results.
I align the bore exactly with one eye with the bolt out so you can see down the barrel, and make a visual comparison of how evenly the rest of the rifle is obscured. To double check it I put the barrel at a 45 degree angle muzzle towards the floor and resting below the bathroom mirror and turn it until the front sight assembly is aligned perfectly with the barrel in the mirror image, then look at the rest of the rifle and see if it is on the same plane as the front sight assembly....try to use this method when no one else is around because you'll look like a lunatic to anyone else...just ask my roomate hahaha...but yeah thats about as scientific as "eyeballing" it can get...it worked just fine for my PSL-54C now it zeros with the front sight post near the center of the "dog ears"
A newbie question on the rear sight. Actually, two questions. One: how does one get the leaf sight out without damaging fingers and two, I got the rear sight in the press area somehow and mis-aligned it. How can I get it back into alignment? I haven't had much luck with getting the pin out, due to the mis-alignment.