I just did this 3 weeks ago on my 1970 442 as the key was falling out of the cylinder when I was cranking the engine. I pulled the cylinder and the code is stamped on the side.
The key was worn out on the end which is what holds the key in the cylinder when you turn it forward. My local locksmith charged $15.00 plus the key charge, all total about $17.00. The new key works great and when compared to the old key, a world of difference!
I had a new trunk key made too and then had duplicates made from the fresh cuts of each key.
If you don't do this, your duplicates will look and work like the old worn out keys.