169Yenko
Before you decide to replace the whole top dash, why don't you cut into the rust area and see how the metal below it looks. You may be able to just do a patch.
I have enclosed a pics of one I did. The first layer below looks kind of bad, but it was solid and I etched the heck out of it when it was open.
Second thing you should do is take a good light to the underside of the dash from the inside. If you see rust coming through there you may have some more to repair including replacing the dash.
I have replaced a upper dash, and made patch repairs underneth to the cowl and firewall too.
Once you get all the paint and sealer off the dash you will find most of the spot welds.
Hope this helps
Mike