Some times free is not free. I had a friend give me an entertainment center. Our 19" TV looked Damn small in there. I bought a 26" and the wife said it was still too small. We ended up with a 32" TV.
if you want to use the MagPul stock get a mthe correct tube. One of my guns has a commercial tube and commercial MagPul stock. It works fine for me so I've just left it.
My point is keep things the way they are and sell, trade or save the stock or change the tube out. If you try to make it fit it is gonna cost you a stock and extension.