Milwroad, since no one else has addressed this, I'll give you my 2 cents worth.
As far as I know, the answer is NO. The only synthetic stock that Winchester ever made for a larger round was the Stainless/Synthetic 375H&H rifle. It was not called a Safari Express and the stock was not set up for accepting the barrel mounted recoil lug found on the Safari Express rifles. You may be able to modify the stock to use one with a barrel mounted recoil lug, though. I've got a Stainless/Synthetic 375 H&H at home but I can't recall if it has the 1 piece bottom metal. However, I'm pretty sure it is a 2 piece. I've got a couple of Safari Express rifles at home as well and I believe the barrels on them match the barrels on the Stainless, so in theory, you could probably make the synthetic stock work with a Safari Express if you really wanted to.
Hope that helps.