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"