Add to your stylesheet_flexible_attributes.css
Code:
#productAttributes {position: relative;}
#wAttrib-4 {position: relative; border: 1px solid #000000; padding: 0.8em; margin-top: 1em;}
#wAttrib-4>h3 {position: absolute; top: -1.8em; background: #ffffee; padding: 0 0.5em;}
#wAttrib-5 {position: absolute; left: 25em; top: 8.8em}
Adjust to taste.

This positions the "last name" relative to the whole attributes container, so if the first name box changes position, they will no longer line up, and you will have to change the last name position to match.

The other sets of attributes can be handled the same way, adjusting dimensions as required.