Iirc, there is a clip that you must grab with needle nose pliers or other implement of destruction and pull out, then pry up on the arm without damaging the paint or body. There is a special tool that makes it easier as they are usually very difficult to get off after 40 years.
Probably too late but KD Tools or someone like that (They provide the specialty tools you see at most chain auto parts stores) makes a wiper arm removal tool that is very simple. It has a hook at one end that you catch under the outer end and a handle that you raise the wiper arm up to. You then lift the arm straight off. The tool basically takes off the spring tension that holds the wiper against the windshield. It's a simple tool, but it saves a lot of frustration and prevents damage to the paint around the holes the wiper shafts pass through.
My question is: what does the center bolt press against? The wiper arms on our cars have closed ends. This would work on later cars that have a bolt or nut that holds the wiper arm in place on a tapered shaft.
There's no substitute for having the right tool for the job.
But I agree with XO...let the windsheild guys do it for you. My F150 just got new glass from Binswanger due to a hailstorm, they were fast, professional, and cheap. I replaced one on a 67 Stang many years ago, and while it wasn't hard and did not take long; the difference between me doing it and watching the pros (guys who have done it hundreds of times) do it was dramatic.