Thanks for the info. Yes I cut the slot for a screw driver with a Dremel. Image, when you say go to the end of the bolt & cut straight down. Do you mean continue the screw driver slot I made? I do have a full size grinder as well as the Dremel.
My idea is to replace the present bolt & nut with a marine grade stainless cap head bolt & marine grade nyloc. Hopefully all will then easily undo allowing me to raise the tank and replace 2 of the planks. At a later date will do the front plank that is not rotten, as it is restricted by the panhard. Need to get the car higher for that. By the way am using Accoya for replacement.
The tie strip nuts came loose no problem.
