Welding is an option....

Another option is to use a tank lining epoxy, using glass cloth over
the weld seams, etc. to strengthen them. Just figure a way to cut
open the tank top and later apply a sister flange for re-closing,
etc.

I use: www.epoxyproducts.com for both potable water tanks and fuel
tank lining. Interlux Commercial Marine Division also offers tank
lining epoxies.