These pages contain a number of pictures of Ford's full-size clay models as the 1961 Thunderbird styling was developed. The quality of some of the photos isn't great but they're an interesting look at what the 1961-63 Thunderbirds could have been!
These early designs from the late 50's expand on the Squarebird look.
This double-sided clay, with variation in the side sculpting, is starting to move towards the Bulletbird look. William Boyer was responsible for much of the early '61 design - he can be seen above with his hand on his chin.