Plastic tanks are great , BUT require more time / effort to install.
Diesel will expand the tank , so it should be lightly mounted ,filled , allowed to expand , then properly installed
Foam in place plus physical mounting.
A couple of years ago Pro Boat Builder had a good article on tank mounting.
Sadly its hard to have a proper sump built into plastic , but a bottom drain might help.
Monel is as always the King of tanks , and worth the effort if the boat is a keeper, and you have the bucks.
GRP does work , but the tanks will need to be very stiff so sloshing will not allow movement / flexing of sidewalls which weakens them.