Since that post I built a 375 RUM/404 improved to shoot the 350gr MK. I ended up with the 350s at 3050fps from a 30" barrel. I'm pretty happy with how it turned out but I really don't shoot it much. I end up shooting the Striker in 284 Win more. Go figure!
Here's a picture of a standard 375 RUM next to my improved version and my 375 next to a 404 case.