You mention you purchased a 111 in 300 wm with bottom bolt release and you want to build a 264 wm so you have a long action with 5.062" bolt spacing. All long actions have the same bolt spacing regardless of being stagger or center feed. You do not mention which MDT chassis you purchased, but does it not have the small slot on the right side to allow for insertion of a tool (flat screwdriver, pin punch, allen key, etc.) to allow you to active your bottom bolt release lever by pulling the tool to the rear? If your chassis predates the slot, might it not be cleaner/easier to mill one into the chassis vs milling your action? Or talk with MDT and see if they would do it for you? While I do not shoot or own an MDT chassis, the one I have shot and all the others I have seen had the slot shown in the pic below and thus accommodated both top and bottom bolt releases.