With a spot meter you should be setting out to examine the luminance of critical parts of the subject and not assume it is overall an "average" scene of Zone V from a singular reference point The method you stated :
is the incorrect use of a spot meter. Examine the entire scene and let the meter determine the SBR and thus the resultant exposure.When I'm feeling lazy, I just pick what I want it to be zone V and meter THAT using spot. Most times, I use it incident if I can get to the subject.
It makes no difference on the camera, certainly not panoramic. Nor strictly does the field of view of the lens. The viewfinder will determine the limits (edge) of metering. But you will be doing more metering for that given (panorama) format. I have done this with my own 6x17cm so I speak from experience.
I don't see how your situation will be improved any by shifting away from the flagship 758.