Below is a great link to all the Ford motor head specifications. It shows you the difference on the 351 Cleveland motors and the "Boss 351". The 4v heads have the largest intake and exhaust valve combination and the port size is also bigger, but the one special difference most significant difference is the combustion chamber. The Boss 351 has a closed or quenched combustion chamber which is 62-64cc vs the open chamber which is 74-76cc. The smaller combustion chamber pushes the compression ratio up about 1.5 to 1 more than the open chamber head.
http://home.comcast.net/~jelerath/mu...master-fr.html