I'm using a standard SA Seekins DBM in a 7 WSM with 162 A-Max and I have plenty of mag space, but if I was doing it over I'd definitely get the WSM system.
Yes and you have two options. One run it in a long action and use 300wm magazines and seat your projectiles a bit longer than you could in a short action.
Or use a short action and tweak the magazine lips to fit the WSM. Brownells sells a lip forming tool that includes the measurments for 20 or so different rounds.