Don't do it.  The main problem in welding any tank is that welding fumes
and particles can collect in the tank and explode even if there are no
gasoline fumes.  Pros run a continuous stream of compressed air into the
tank while welding.

Leave this to a pro, most radiator shops can clean and repair tanks for
a reasonable fee, it's not worth the risk.
--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia w/2.1
'65 MGB (Daily driver since 1969)
'74 MGB GT (Restoring sloooowly)
'01 Toyota Tacoma (Something I don't have to fix!)

Personal mechanic for:
'70 MGB GT (Daughter's)

Assisting on Restoration (and spending OPM):
'72 MGB GT (Was daughter's, now son's)
'64 MGB (Son's)

Stripped and gone but their parts live on:
'68 MGB, '73 MGB, '67 MGB GT