A 300 WBY Mag would be a long action caliber. Please look at our part number 80700, we have hole spacing measurements listed out. Compare those to your receiver to confirm fitment. This mount is ambidextrous and will work on both right and left handed rifles.
This mount should fit your gun. Please find this part on the EGW website, we have the hole spacing measurements listed out on that page. Compare those to your receiver to confirm fitment.
You will most likely need a rail meant for a 9 lug action. Please check your center spread to make sure this rail will fit your application. The center spread is the distance between the two middle holes, center to center, on your receiver. Your center spread should be 4.485". Those 9 lug rails can be found here:
https://www.opticsplanet.com/evolution-gun-works-hd-weatherby-mark-v-picatinny-scope-mount.html
Choosing between 0 MOA and 20 MOA is going to be dependent on what kind of distances you will normally be shooting at. General rule is that if most of your shooting is over 500+ yards you will want to go with a 20 MOA rail.