Hi Jahn,
Based on my experiance the 1980's M80 model with the maple top is heavier than the 1970's model. The 1980's model is thinner but heavier. I didn't find the the 1970's model to be that heavy. Over the years I have owned 3 different 1970's models (2-black & 1 walnut). I still own the walnut stained mahogany guitar. The 1970's M80 also has a very slim neck.
Interesting thing is that when I was younger I really liked the slim Guild necks. Now a days I go perfer bigger necks. My hand gets less fatigued with a larger non chunky neck.
As a matter of opinion I would say that overall the 24 fret neck of the S70, S300, S25, S250, S275 and M80 (1980's model) guitars are some of the best necks Guild ever produced for electric solidbody guitars-quick and fast with low action.
As I slow down I like the Bluesbirds and Nightbird necks. I also favor the Starfire 4/5 neck feel.
Looks like we can have a new thread about necks.
M