Take the 7 Dakota
(or 7 LRM) and neck it down. It is a bit overbore but you don't have to load it to max. Get some slow powder like RL33 or RL50 and shoot the 160's in it with a moderate charge, or zip the 140's down range at 3400 +, maybe 3500, maybe 3600 with RL33.
That said, I believe Gunwerks is working on a 6.5 LRM which is a smaller cased 7 LRM necked down. Might be right up your alley.
The WSM and Sherman are good choices but nothing different than the 264 WM in performance. In fact the WM has a bit more capacity, but they will give you 3200 fps with a 140.