When we lived in Michigan my dad bought a 1976 Caprice station wagon with loads of rust as a winter beater. The gas tank had the same sort of leak you described, so he slapped on some duct tape really quickly and sprayed a generous amount of undercoating on it. It never leaked after that, but I wouldn't suggest doing that especially inside the cab. Eastwood has some products that clean the rust out of gas tanks, etch the surface as a prep, and then coat the inside to seal off pinhole rust. I don't remember the exact products, but I'd give their site a look if you want to keep the tank inside the cab. Here's a link to a guy using it on his Ford truck:
http://fordification.com/my67-p36.htm
