Did this repair on a Norton tank last week. It was in a seriously bad state.
I had to cut out the tunnel and beat, wheel, and shrink all the dents good enough for re chrome. And it needed to be air tight!
The WD B40 used the same tank form as a DBD Gold Star. The differences being: no badge indents, no breather tube boss, some differences in the base and tunnel, and a standard push-and-turn filler neck. We had a very accomplished chap who used to modify them. That was a nice little earner. He was also a dab hand at repairing bent tanks.
I seem to remember he described one of the techniques he used, as being a small hammer on a spring arm, that could be inserted through filler necks to raise smaller dents.