Uli- there is- or should be- a round synthetic rubber gasket between the sender and the tank to contain splash leaks. They're cheap and available from all the U.S vendors and probably from Roland Jaeckel in Germany at detomaso@gmx.com.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.