I don't think anyone has yet answered your original question re the differences between the 2V & 4V heads - The major difference is that the ports in the 4V heads are much bigger than those in the 2V heads & as a result have slower air flow (but higher volume) through the ports, resulting in poor low RPM torque, but great mid to high RPM power with the right combination of cam, intake (carby & manifold) & exhaust. The 2V & 4V designation comes from the original intended use of the heads - the 2V heads were originally intended for use with a 2 venturi (or 2 barrel) carby & the 4V for a 4 barrel carb. This doesn't mean that this was the case in production & doesn't restrict you to using either carb. In fact, for a street engine you are probably much better off (as suggested in another reply) using 2V heads with a port job, concentrating on the Exhaust ports, as it is widely recognised that the Clevo heads have a great intake port design, but a relatively poor exhaust port design, which ultimately restricts the power the engine can make as what comes in must also go out! With regard to the open & closed chamber question, the major difference here is the combustion chamber volume & if my memory is correct (I used to have an XB GT with "relieved" closed chamber 2V heads) the compression ratio of 2V closed chamber heads on a 351 clevo is around 11.5:1 (much too high for petrol). if you want to have a higher compression ratio than open chamber heads (approx 8.9:1 on a 351 clevo I think), you are best off getting a head specialist to relieve the chambers (ie. remove some material to increase the volume) to achieve the ratio you need for the cam, carb & exhaust setup you are going to use. (cam lift & overlap has a big effect on cylinder pressures & can make a big difference to what static compression ratio you are able to run without detonation).