I had the same issue with those mags. The plastic baseplate is a bit wider than the factory metal one.
The solution was to sand down the sides of the baseplate until the mag seated easily. You dont need to do anything to the front or rear of the baseplate...just the sides. If I remember correctly it was something like .020 that I had to remove from each side.
You should still do a quick measurement of the width of the stock metal pad vs. the plastic ones to confirm how much material to remove. Measure, sand, test fit...repeat.