With .308 ammo costing what? five or TEN times as much as 7.62 x 54 ammo, I don't see how a difficult & expensive conversion will make up for the price of a few mags. Keep in mind the PSL mags come & go. Don't be surprised if half way through your build they come back to four in a pouch for $100.00.
Mutt builds are cool no doubt. I have all the parts in a box buried in the kit pile to try a PSL in 30-06 using BAR mags.......someday......if I live that long!..Heh...heh
Either way, good luck!