Good article. Your airplane has an older style fuel cap and flange
arrangement than the original poster, and his skins are more difficult
to remove. His advantage is not having that recessed pit around the cap
that collects water and dirt; the cap flange is flush with the wing.
His disadvantage is getting that gasket back into place while replacing
the skin; a bit of fuellube will hold it in place. I suspect his tank
skin is being hard to remove because someone used Form-A-Gasket or
something tough to seal the gasket last time, and it's stuck good.
172s used all-brass floats by the time our L and M models came
out. They're OK, but we had one fail because the arm stop was
misadjusted and the float was tapping the top of the tank when the tank
was full of fuel. It wore a hole in the float, which filled and sank.

Dan