Easy -
1) Find someone who can weld, if you can't.
2) "Stop-drill" the crack - a 1/16" hole drilled in at the end of the crack will be enough (do this straight away, so the crack will stop expanding.)
3) Using a burr and a die grinder, grind out a V-notch along the crack. Both sides, if your welder can reach it.
4) Using a wire wheel, strip the paint about 1" side, centred on the crack.
5) Weld the crack and stop-drill holes. I know - you're welding up a hole you just drilled, but the hole was - and is - important! Without stop-drilling, the crack can start running again.
6) Grind smooth and finish. Don't leave the thing primed when you are done - primer is porous. Either apply a colour coat to the area, or use a decent undercoating.