I finally got down to work after avoiding the inevitable. I decided to tear out the old leaking diesel tank, and I am really glad I did. After struggling to move the oddly-shaped aluminum tank out of its well between the stringers, I found about two gallons of fuel in a puddle OUTSIDE the tank! No wonder we had such an awful smell all the time!
Now, after trying unsuccessfully to maneuver the tank through the lazarette opening, I realize it must be cut apart.
So here's the next question: will the diesel fumes ignite in the tank if I try to sawzall it apart? I will try to drain it empty of diesel as much as I can, but is it dangerous to make sparks? I know it's not gasoline, but better safe than sorry. My internet searches on the topic are inconclusive.