I have been repairing/welding up petrol tanks for over 50 years and never had a problem, you just have to be careful and flush them out very well. And MF you are right after I have flushed the tank before I get close I always wave the oxy above the filler cap, that way you cab be sure you can get up close to it to work on it.
AVB when I say silver solder I am referring to the 5% silver type that is used for welding copper pipe, not the 50/50 soft solder used on tin, this stuff is not suitable for repairing petrol tanks but many have tried, sort of works sometimes
