The difference between open and closed chamber heads is the volume of the combustion chamber. On open chamber heads, you have a lot more combustion chamber volume. This results in a lower compression ratio. Generally, the higher the compression ratio, the more power you can get, -- up to a point!
OK now, with that prelude out of the way....
All US-made 351C-2V heads are open chamber, or low compression. All 351C-2V engines used flat top pistons.
The difference with the Australian 2V heads is that they are closed chamber, or high compression. They generally are not seen too much here in the US. If you do see a set, they are generally selling for more than closed chamber US-made 4V heads!
The 351C-4V heads have ports and valves that are almost too big for the street. This results in low velocity in the intake and that results in gas coming out of suspension, bogginess on acceleration, poor volumetric efficiency, (bad gas mileage), etc. At higher rpm ranges, the bigger passages of the 4V heads helps to flow more air/fuel mixture than the 2V port sizing will allow -- more flow=more power! This low-rpm sluggishness is one of the reasons that Ford put a high stall torque converter behind the low-compression 351C-4Vs -- to get the engine up into the higher rpm range faster!
351CJ heads - open chamber 4V heads.
Then in 1973-74 Ford shrunk the valve size of the 4V heads down to the same as the 2V valve size to try to boost intake velocity a bit.
Keep asking questions. That's the way to learn!
Milo