welding a dry tank is fine, we used to fill them with sand and weld them years ago but as said above if they are dry they don't blow up, it's usually if people rush it and don't do it properly, when you get the tank out you may find it's all rusty inside anyway so i would get it out and see what you're up against before speculating.
i had a hole near the top of the tank i rubbed down the area to get it clean and rough then used some liquid metal that was sold for fixing petrol tanks and put a bit round a self tapper screw and screwed it in the hole and then a blob over the screw. http://www.amazon.co.uk/Pro-Seal-Instant-Petrol-Radiator/dp/B001JT5OSO/ref=pd_sim_sbs_auto_2 no leak so far
