Fuel tanks of that era were often made from 'Terne Plate' - which is steel coated with a lead+tin alloy. (later terne plate was steel coated with a zinc/tin alloy, nowadays, terne plate is a type of stainless steel with a tin coating).
If repairs are to be carried out by welding or brazing, then this coating needs to be removed in the vicinity of the repair - and appropriate safety precautions employed to deal with any fumes.
So, the usual method of repair is to use solder for small holes and soldered-on patches for holes. (this is why I mentioned finding a radiator repair w/shop).
Always clean the tank before any cutting, sanding, grinding, welding, brazing or soldering is carried out.
Ideally the tank should be steamed out thoroughly after draining. The biggest danger is the risk of an explosion caused by igniting any build-up of petrol fumes. Even after the tank is drained and dried out fully, it appears that fumes can still build up again. Some say that petrol soaks into the pores of the terne plate and re-emerges later, but idk....Petrol can also linger in any sediment or rust inside the tank.
Some suggest positioning the tank so that the area to be heated and repaired is uppermost and to fill the remainder of the tank with water, to minimise the area/volume in which fumes could accumulate. But this isn't always possible and is awkward due to the weight of water/shape involved.
I'd settle for thoroughly rinsing out the tank with a detergent/very hot water mix. Remove the sender unit and drain plug - anywhere that petrol may linger.
Soldering can be done either with a very large electric soldering iron, an old style heavy soldering iron (heated by a flame) or a blowtorch.
You can use either 'Baker's Soldering Fluid' as a flux or a tinning paste. you can get these and suitable sticks of solder (not roles of solder wire) from a plumber's merchants or DIY/hardware store. For affixing patches, clean the edges of the hole and tin with solder, clean the patch to be applied and tin the edges with solder, then fix the patch in place by a process called afaik 'sweating' - heat the patch and tank until the solder on both melts and feed more solder into the joint to ensure a complete bond without gaps.
I saw this very technique being used recently on tv. Iirc, it was in an episode of Wheeler Dealer's with Mike Brewer and Edd China which is on 'Quest'? Channel, in which they had a rusted-out petrol tank from a Jensen Interceptor? repaired by a specialist. You might be able to locate this program or a suitable youtube video to see the technique in action.
After you've repaired the tank, it might be a good idea to seal the tank inside with one of the products sold for this purpose, to hopefully avoid future problems.
I've worked in both a car workshop and a truck workshop where people became complacent around petrol fumes and paid a heavy price. (Exhaust silencer being welded, exploded/ petrol tanker blast, tank had been steamed out for several hours, tested ok to weld, but job then left for a day, fumes had built up again!).
Please be very careful at all times and wear eye protection/face shield
AL.