50 WSSM now that I'd buy!
Be careful what you wish for, as B&M's 50 Super Short Magnum exists and if you have a Winchester/USRA 70 WSSM action and around $1K, SSK will build a B&M Super Short in .458", .475", or .500" bore for you.
www.b-mriflesandcartridges.comOr, you could simplify your life and just do a WSSM in 500 S&W.