If its washed repeatedly,hot water helps. Then run compressed air in tank until it is bone dry should be zero risk of kaboom. Let your nose decide. It shouldnt smell like gas.
This can still explode because the gas gets into the pores of the metal and when you start to heat it it's released. You might try filling it with water and then welding it that way there is no room for the oxygen to be in the tank because it's got water in it. I would put an exhaust pipe to it fill it with engine exhaust and weld it.
Actually I would take it to a welding shop and let them do it if you're not sure how to proceed.
