Early A windshield replacement. How hard???

Here's a link that shows the rope method mentioned above. (although it's on some off brand model) I've done a lot of 'em with that method back in the day and it works pretty well. Lube the gasket with some non-pumice Go-Jo to make things a bit easier. It's also easy enough to just set the glass in place and pull the gasket into place with glass tools. These cars are really pretty easy since a lock strip goes in to fill the slack. If you are careful and take your time you should be able to accomplish this easily yourself.
http://www.route66restorations.com/1.html