Rear subframe welding - do I have to remove the fuel tank (diesel)

The bottom of my rear subframe behind the tow bar mounting points is looking a little sad along the bottom face - it looks as though it has actually split. On the other side (forward of the towbar mounts) it's solid one one side and the other side has already been repaired.

It's quite a long way away from the diesel fuel tank so is it safe to mig weld repair plates onto the chassis legs without removing the tank?

I take it from subframe you mean chassis?

If so and I was you I would drop the tank, once you start cleaning you will undoubtedly find more rot and holes.
You'd be as well doing the job once properly rather than little patches now and then

Sorry, yes, chassis - I was discussing Mercedes subframes earlier and got confused
It looks as though the chassis forward of the towbar mounts has had a lot of work done on it already with at least 2 sides being plated so I really don't want to drop the tank unless I really have to. It just seems that for some reason they didn't tackle the rear section - maybe because 3 sides are solid and it's just the bottom that appears to have cracked.

should be safe to weld as long as your not a splatter welder however as biggeeeeeee said best bet would be take it off tackle it all in 1 go