I put a McMillan Super Grade with magnum fill on my .375 H&H Stainless Classic (very similar size and weight to Safari Express). Seems to work fine with the iron sights.
I'm curious about the McMillan Safari stocks. Does the butt on them have a bit more surface area?
I really like the newer South Carolina M70 Safari Express stocks. They are like a Supergrade stock except more broadly proportioned, including surface area at the butt. An excellent design that I hope McMillan will duplicate. The bigger the cushion, the better the pushin'.