If your tank has "some holes in it" it probably has several more just waiting to pop thru and tiny pinholes which won't drip gasoline, but will let it seep and show up as a wet area under the tank. I have repaired a number of leaky old tanks using muratic acid and gravel sloshed around inside to get it clean as possible. Then used MarineTex, a 2 part epoxy like JBWeld, on the outside of the tank. I think MarineTex is better than JBWeld, but also more expensive. You can get it from any boat dealer or the marine mail-order dealers.

There is also a product called KREEM which has been around the motorcycle catalogs for a long time. Costs about $25 for a 2 part kit of tank conditioner/sealer. Works from the inside and provides a plastic liner. I have used it successfully on a tank that was not too badly rusted that I wanted to save for its antique value. Dennis Kirk sells it and probably most other mail-order cycle places.

If you're going to weld, make sure you are well versed about welding gasoline tanks...nuff said.