Do the best you can with what you have by 9/15, then file an amended return if it turns out wrong.  

You can always use zero as the cost basis.  But there's not much risk in using something higher than that.  How many square feet in the house?  And what was the range of price per square foot, back then?  Use the low end of the range.  Newspaper back issues are online, and they still had classified ads back then.

Don't tell your client why it can't be done.  Tell him how you are going to do it.