I didn't see where you mentioned the cell had ever had fuel in it. You could install a bulkhead fitting I suspect elsewhere and just plug and epoxy the partial threads left over and skip the welding solution.

The fuel cell link sounds/looks decent, almost priced too low, makes me wonder as if it's up to expected standards.

Who will go first? grin

Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.