Rather than using duct tape, which will certainly get in the way and who knows what will happen as you're applying all that heat to it:
1. Take a good photo of the letters showing where they line up in relation to the Chrysler logo/wings and panel edges
2. Make a pencil rubbing. Tape a long piece of paper over the letters and rub your pencil over each letter.
3. File the letters, rubbing and photo together. Write down measurements as an added guide.
4. Later, during restoration, use the photo to guide and line up the center letters, tape down the paper, and apply each letter.