A name brand wrench would be best but there will always be a bit of play.
If you have room you could use a pair of vice grips to close the wrench to reduce play.
Like Scott06 said, if it's the line why not cut it and use a box end wrench?
Have you checked the thermostat to see if it's stuck open or opening at the marked temperature?
There's many honda motors running in northern climates without issue.
Someone may have put the wrong thermostat in.
Not sure what you mean by a "higher temperature rating"
The carb rebuild kits for 1968-1982 2.5 and 3.0 liter should all work if it's the original carb.
Mercury # 823427A-1 Sierra 18-7746 or 18-7070
I just measured the base gasket from my 823427A-1 and the mounting holes in it are 3.28" x 1.91" and the bores are 1.487"