Beneagles, I think to be honest to do that properly you need to take the tub off and fully asses this. It's pretty bad looking and it's a pretty important crossmember as it holds the A-frame.
Tub off probably sounds like a real pain but to be honest it will make things so much easier, I would be tempted if it was mine (and I wanted to save it) to cut out the whole crossmember and large sections of the rotten chassis and replace the lot. If you call Richards chassis or someone like them they may even sell you the crossmember section if it isn't already available as a section to buy anyway.
Nice repair work there ARC.
I notice you have fully seam welded along the bottom of the crossmember to the top of the chassis rail so water and muck can't just run down under the metal work, terrible design I think, BUT I guess it also stops a water pocket? I am looking at that thinking maybe I should clean mine up, weld it, then put a 12mm drain hole either side?!?!