Oxy-acetylene welding is also a viable proposition; I used to hire a set in the early days.

Or a MMAW (stick) welder running 1.6mm rods will work well, if you are used to position-welding.

If that is the only repair you have to do, it can be done in-situ; if there is more rust it may be worthwhile to remove the bulkhead and set it up on trestles.

I have repaired a couple and like Gav I used a MIG,

Good luck,
Cheers Charlie