I had the exact same leak on my 66 Safari, and I tried everything to fix it, from marine epoxies, to goopy thread sealers, even fiber glass. Absolutly nothing sticks to the plastic tanks. I however did not try to heat weld it, since it was right at the thread joint. It was making me crazy, because I go to the dessert and dry camp a lot, and rely on my water for backup drinking water.
what I did do, was install a brand new tank, which took a bit of engineering to find one that would fit. Rod at C&G Trailers was a big help to me (800.662.3790). I just gave him the dimensions, and he had one custom made. It cost me about 150 bucks, but now I have a brand new (clear) tank that I can see my water level through the plastic.