On my old 1984 300d I drilled into the fuel tank from the trunk to mount a bracket, not knowing the fuel tank was there. Didn't realize what I did until I opened the trunk and found it full of diesel. Fixed it with a sheet metal screw slathered with JB Weld.

Don't know what kind of leak you have but I would recommend JB Weld. It will work without draining or removing the fuel tank.