The only problems I had with the ARMS receiver cover was trying to get it to fit the first time. When they say it fits tight, they aren't kidding. I"ve had it off several times, and at the carbine class I took, I could still ring a 12"x 14" steel plate at 100 yards pretty easy, so it didn't shift zero much, if at all. And I"m not too great of a shot either.
Mine is an STg 58 kit, I swapped out the dust cover, buttstock, pistol grip,T-48 square trigger guard, the charging handle for a metric folding style, inch style selector, FSE internals (H,T,S) and a Stoll type muzzle brake.
My only complaint would be, that as shooting as a lefty, I do occasionally bump the safety up to safe. I do like the angle of the SAW pistol grip a bit more than the original FAL one.
Someone on FAL files had a picture of a FAL retrofitted with a SAW forearm. I cant seem to find it right now tho.