I recently aquired a 105gal slip tank for my truck. It was leaking in a few spots so I decided to fix, then mod. I wanted the top part to sit lower than the bed rails so I can run a tanoe cover If I want and not have to pull the tank. I cut about 8.5in off the top and replaced the top with a new piece of 11ga. I had to cut the filler out and reweld it into the new top. It now is a 82gal tank but still over 100gal capacity when adding my trucks tank. Oh and before you ask, before I cut the top off and did any welding, I put 5# of dry ice and 15gal of water to purge the fumes. worked great.
I will post more pics later, but here is a taste.