builders laser for striking horizontal lines and leveling / centering end to end, set of horses that can be shimmed and leveled from end to end. plumb line or level, a smooth floor to strike center an layout lines to 'loft' onto the logs. a plywood box that can be strapped to the log to act as a base for router / layout / chain mortiser / etc. flexible rules and discarded metal strapping also come in handy.
applying ply to the ends will require you to level the timber and accurately square cut the ends prior to any further layout... you can also just use sticks screwed to centerlines on each end as required.
