Fuel leak on/around gas tank

1989 Chevy Caprice, 4-door, auto transmission, a/c, 305V8, 165,000 miles.

Notice strong gas odor, but only after driving. Comes from the back. Looked under car (after driving), (only while crouching by the side of the car, did not go underneath) -- no gas on the ground. But noticed splotches of wetness (i.E, gas) on the tank underside, toward the front of the underside of the gas tank, around the middle -- extending from the leading edge. It looked like it had been seeping from somewhere on the upper side or the front side of the tank to that spot. I imagine there's a leak around a gas line fitting, or any possible wiring penetrations.

What could it be and how easy/expensive is it to fix if I take it in? Is the fuel line welded to the tank, or is there some gasket/fitting/rubber part used for interface? (Mechanics non-committal on the phone, claim it could cost as much as $800-$1,000, if they have to also replace the brake lines, which they claim may be nearby; that seems way off the wall).

Thanks. John W.