This is something that I only know how to do with the heads off. Visually look at the chamber, small heart shaped chamber like this its a 4v closed:
Open round chamber like this its a 4v open:
Which is better is always debatable, but on an otherwise stock bottom end 351c, the 4v closed will make more power since they smaller chamber increases compression on the motor to about 11:1. On a BUILT motor, they both make great power. I ran the closed chamber heads on my stroker and since I broke them, I now run open chamber heads on my stroker. It was an easy choice for me, I had to get pistons custom made either way so I bought the less expensive head and andhad the pistons machined to get me the compression I want.