Not sure about the AR-10 mags in the MAS-49 magwell. M-14 mags should have no problem fitting as they are rather compact for a .308 mag.
Wish I had some pics of those conversions. They are long gone (sold to AR-10 enthusiasts actually).
The only trick part is the stock and pistolgrip adapter. Mine were made of milled aluminum but I believe that a fiberglass item would do the job especially for fellows without a milling machine. The AR grip fitting was made of 3/8 inch steel and welded to the trigger guard (after a bit of re-shape with a hammer). However this trigger guard assembly was then bolted to the action (through the above aluminum block) via the original MAS rear tang screw hole in reverse direction from the original screw.
A G-3 or AR flash hider makes the barrel look authentic.
While I milled my own steel scope mount and it was a PITA. Get a French original. They slip right on, no sweat GI.
Oh yea, I replaced the plastic door knob on the cocking handle with a small knurled steel sleeve.
VD PS... think I'll look around for another MAS-49.