rot down side of windscreen

[poptart]

if you look at the gallery, poptarts bitch, you will notice that there is some quite bad rot around the top of the windscreen. When i eventually get my arse in gear and start fixing things I plan to spray her after sorting the bodywork. But what i'm after is some advice. Should I (after getting the screen taken out!) cut out the whole section from the rain gully bit down to where the bonnet starts and weld in a new bit, or grind it out (whats left) and try to weld bits to it? was worried that after doing the latter the winscreen might not sit right and crack. I have seen underneath it last time I had it replaced and its quite crusty all the way down, and I didn't get any guarantee on my screen cos of it