I should have welded the lug on the rear about 1/8" higher, and shaved the front lug off and welded a bead back on 1/8" higher. It works the way it is, but would have been less trouble, less grinding, bending, profiling, etc... if I had done it that way. Then the mag would have sat 1/8" lower in the receiver, which would have been perfect. Might have even dropped it 3/32" in the front.
For the front lug, I would grind the factory one off, and weld a bead across the front right where the top edge of the original lug was. This should drop the front of the mag down far enough. You can see in the middle pic how much material I had to remove at the front of the mag to clear the underside of the rails/locking lugs. It was roughly 1/2" down, and 1/2" back, with a gentle radius. I wanted to leave some of the bullet guides on the front sides of the mags. Push a cartrige out of the mag, and you can see what I'm talking about. There are little pieces that protrude into the path of the bullet to gently guide it toward the centerline. I left just a tiny bit of these, just in case. I don't really think they are needed anymore, and I may shave them off, but until I take her out and fire it with these mags, I'm leaving them.
On the rear I used a PSL rear lug that I drilled off of a couple rusty mags. I lined the whole assembly just a hair above the top of the feed lips on the MG13 mag. Maybe .020" above the feed lips. BUT that's where it needs to go at least 1/8" higher on the mag, so that the mag will sit low enough to clear the bolt without much filing to fit. Mine was hitting the ejector tip so hard that I had to roll the lips over more to get some more clearance, as well as thin the locking lug on the rear thinner than what I would've liked. It's still nice and solid, and it's not going anywhere, but I would have rather it been thicker.