Ray
If this is an old tank It may have been brazed, and welding over braze material is always a problem. If it is, try extra cleaning on both sides if possible.
If this is the case and you can't prep both sides, than in the end it may need to be rebrazed
When I was a kid I use to work in a shop were we occasionally did custom machining and welding for a bike builder down the road. Used to get lots of used gas tanks in for repair or modification. We used to wash them out several times with soap and water then fill them with water and let them sit for a couple of days. After a couple of days the gas smell would go down alot, then we would weld them and never had any cracking problems. Like mentioned before gas is a hydrocarbon and will acutally seep into the steel itself and will sweat out when heat is applied. I don't know if that would cause the cracking, but I've seen stranger things. However we use to soak the tanks as a more of a safety precaution than anything.