I need to weld in a small piece on the gearbox mount. Which right next to the fuel tank. Just as a precautionary, I drained the tank and filled it up with water. Is it still unsafe to do this procedure?

is it easier to take the tank out?????

tank out = painful job
fuel fumes are the thing that goes bang not so much the liquid..
be bloody careful with what ever you do..
safest to take out tank really..
i dont envy you mate..

water = rust, eventually. make sure you dry it reallllly well afterwards.

see if you can get a heat shield or blanket or simular

http://www.heatshieldproducts.com/torch_blanket.php 

even a leather welding apron would be better than nothing

Hmmm, Whats needed to remove the fuel tank?