The 1975/1976 Fuel Tanks have the Fuel Sending unit located half way up the side of the tank, not on top like the 77's and 78's. The earlier models are more prone to leaking from the sending unit O-Ring because it is below the level of the fuel when the Tank is over 50% full.
My sending unit was leaking around the 3 electrical wire connections that exit from the fuel sending unit. There are some small compressed O-Rings on the inner side of the sending unit. These O-Rings tend to dry out and crack with age. Unfortunately, it is next to impossible to replace them as they are Rivetted in place. Only leaked when tank was more than 3/4 full. Might be something to check.
The 1976 Fuel sending units are nearly impossible to find. I've been looking for NOS over a year.
What I did for a fix was to remove the sending unit from the tank, Then I thoroughly cleaned all around the terminals ( inside and out ) with a Dremel and Wire Brush. JB Weld makes an excellent product called " Water Weld " . This is about the only epoxy that will seal fuel tanks. Regular JB Weld will eventually dissolve in Gas or Diesel. This stuff will not. It is Gasoline, Diesel, Oil Hydraulic Fluid and Chemical resistant. It will cure under water... which, I guess , is why they call it " Water Weld ". After cleaning around connections I laid a bead of this stuff on the inside and outside of the connectors. This seems to have stopped the leak permanently. I can now fill the fuel tank up and no more leaks on the ground. I'm still looking for a replacement Fuel sending unit though. As with any Epoxies or Glues, proper surface preparation and claening is essential.
http://www.jbweld.com/products/waterweld-epoxy-putty