If I frame mount the snowplow, I don't think the snowplow could be too heavy. Adding about 750 lbs of ballast to the 3point, loaded tires with chains would be about the best I could do to prevent swinging the tractor when angling the plow.
I know 8' of plow vs. 7' would increase the leverage of the tractor to side swing.
I know 8' of plow vs. 7' would increase the leverage of the tractor to side swing.