I have a copy of a PDF chart showing evolution of all Mopar body lines. It shows 1960-64 fullsize as "1st generation unibody", without a letter designation. The information apparently came from Diran Yazejian, a designer at Chrysler from 1959-96. I found an online copy here:
https://fcagroup.pl/wp-content/uploads/2015/05/Early-Chrysler-standardization.pdf
When you're trying to figure out parts interchange, it's more important to know which years are mechanically the same rather than similar styling. Mechanically, 63-64 should be close to 60-62 Forward Look cars, even though they may be styled differently.