Arc and spark won't be the concern-no issues there. Hell, I have a torch burn in my plastic tank! What IS an issue though is clearance-yes, pull the tank and have some room to work properly. Not worth it trying to do it with the tank and fight everything along the way. Pull the tank, get the welds in good then be done with it. Good time to clear all the gravel, dirt, debris from the tank anyway. Just be sure to plug the fuel lines good and get them out of the way, then weld away!
Best of Luck,
Mike
Best of Luck,
Mike