I can highly recommend POR-15 tank sealant, it is a US product so you will have no trouble finding it. I have used it a few times on old tanks and it does a great job and, once it cures, it lasts forever.
Once it is cured it is like having another tank inside the tank. It seals splits and pinholes so I think it will do a good job of reinforcing the metal. You could also use a little underneath the tank with the reinforcing patch that they supply with the kit.
You do have to follow the process carefully for best results. using the cleaner and primer first.
POR-15
