I too had the same problem on my 58, originally I just wanted the perished rubbers to be replaced but when the windscreen was removed the corners were visably riddled with corrosion where water had been sitting there for probably years. From memory it cost £200 approx to weld new L shaped bits of metal in and respray. The L shapes are the bits on the inside i.e. the demister heater vents, the garage I used manufactured these themselves.
The biggest problem was getting the windscreen back in, apparently mine was slightly warped i.e. everytime one corner went in the other popped out. Eventually I just bought a new one (£50 if you look around) and this went in fine. :wink: