I would think maybe your insurance would cover it. Can't remember the name of the company but they replace all windshields and work with insurance. Can't promise they go that far back but they fixed my 87 F150 and didn't cost me anything. Even came to my house to do it.
There are a lot of posts on this site regarding windshields. My suggestion is to find a shop that has done some early Mustang windshields as Craig has pointed out. These windshields are notorious for leaking, so experience with them is important.
For what it's worth, I replaced mine myself with successful results following some online videos, and had a friend as another set of hands to help. I wouldn't say it is terribly hard...just time consuming to get right, especially if it is your first one, and it's going to be a bit messy as well.