The magazine on a GEW needs little (if any) work to make it function with the 458 Win. The feed lips of the receiver are what needs the work. You need to widen them and move the shoulders forward so the wide 458 cartridges are not "pinched" into the center of the mag.
I have done about 20 of them in my life and the work is a bit slow going, but not hard to do.
I have used a Dremal tool to grind the majority of the metal away where it needs to be relieved. Next you shape stones or even bars of steel to glue metal cutting paper to, as the finish stage of the work is done. It takes time but it's not hard to do. The ones I have done were as sooth to operate when they are done as the standard 8mm was before I started the work. You will also have to widen the feed ramp on the receiver, but only about .060" per side. Again it's easy to do with a Dermal with grinding burrs and then Cretex to finish.