I have a Win M70 EW in 7mm RM. I use 2 piece Leupy STD bases and rings to mount my Zeiss Conquest 3.5-10x44. Works perfectly. Indeed, one of the things I like about the Leupy STD base/ring combo is the ability to adjust windage with the base set screws so I can sight in with windage left at mechanical zero. I've had excellent results on a Rem 700 SPS-V in .308 Win on which I have mounted a Vortex Viper PST 6-24x50. I think a 20 MOA Pic rail is a good idea for a one piece if you're going to shoot regularly out to 1,000 yards and beyond but the STD set-up is good out to 800. Leupy offers a one-piece STD base for a Win 70 (their prod code is 51736 for the matte base one-piece LA base).
Good luck.
FH