HeadsCleveland heads were produced in several variations. Most common is the two-barrel (2V) casting with 74-77cc open chambers, 2.050-inch intake, and 1.659-inch exhaust valves. These are easily identified by their open "bowl" chambers and D0AE-E or DOAE-J casting numbers. This is a good street head because the port size is more conducive to the low- to midrange torque we want on the street. Aftermarket intake manifold availability is better with these heads as well. The 2V head was used not only on the '70-'74 351C, but the 351M and 400M engines as well, which explains availability.
Four basic types of 351C-4V heads were produced-closed chamber, open chamber, Boss 351, and High Output. Much of it has to do with compression ratio and mechanical lifter use.
Early 351C-4V engines were fitted with closed-chamber (61-64cc), large-port heads. This is an excellent high-revving street/strip head for those who want iron heads. In 1973-'74, Ford went with a 73-78cc open chamber in the 4V head, virtually identical to the 2V head. This head is easily identified by its large ports and 2V-style open chambers. It's the least desirable head available, but sometimes it's all you're going to find.
The '71 Boss 351 head is extremely hard to find, thanks to adjustable stud-mounted rocker arms and closed 64-67cc wedge chambers. The '72 High Output that came a year later has adjustable stud-mounted rockers like the Boss 351. But larger 73-76cc open chambers make it less desirable than the Boss 351 head. The truth is, aside from the stud-mounted rocker arms and large ports, the '72 High Output head isn't desirable outside of a restoration.
351C-2V '70-'74 351M '75-up 400M '72-up