Dave is right. It's been 35 years since I played with 351 C's, but those are called open chamber (2 barrel) heads. Date code shows around 1971. The 4 barrel heads are called quench chamber heads, and when you look at the shape of the valve area it is not round like yours.

Yours aren't bad heads, but just not the revered 4 barrel heads everyone used to be looking for.

Don