Hi Chris that last picture there you posted is of a local owner here in Ireland ..
He has tryed welding it with aluminium rods twice I believe and both times it leaked .
It seems there s at least more than a few holes . He is now in the process of doing an engine swap .
I have been looking into this with great interest to as my valley had holes in the trenches to .. thankfully they didn't go all the way thru . A friend recommended a company in cork to try a new process of aluminium welding .
I sent the car down with valley completely stripped and he prepped the entire surface and trenches of the valley by giving it a etch of soda blast and then used a special gun to shoot molten molecular aluminium to fill the holes up..
I done this 4 or so years ago and still holding up well fingers crossed !..
It's another option dunno if anyone else has tryed it ..
But I definitely believe there has got to be a way of fixing the rot
Cheers
Shane