Jaguar mk2 1967 3.4
Which radiator cover to choose?
Which thermostat to choose?
between various measurements (lbs)
If the system is in good condition then the 7lb cap will give you a more efficient cooling. Effectively the increase inpressure allows the water to absorb more heat.
Boyle’s law is a gas law that states that a gas’s pressure and volume are inversely proportional . When the temperature is kept constant, as volume increases, pressure falls and vice versa.
In this case the volume is constant, the increased blow off pressure means it has a greater range to move with the increased temperature
I am sure someone will give a better exlenation of the physics 
On the thermostat the answer is probably neither of the above. The ones shown are a ‘normal’ type and you want a ‘sleeved’ one that will open and close the engine by pass system. Effectively this isolates the full cooling system from the engine as it is first started making it warm up a lot quicker. They are much more expensive and you will have to do some digging around to find one.
As for temperature - normally 74 degrees unless you are in a particularly hot climate
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It it a real 3.4 or a 340.
For the thermostat there is a real difference, as the 340 has a Straightport head.
Post a picture of the inlet manifold, then we can see what thermostat to use.
Not all 340s have SP heads.
The boiling point of water changes with pressure, and is only at 100°C when at sea level. If the pressure is lower the steam can escape the liquid more easily. With a higher limit (7lb) there is more headroom before it boils over. Not that it should.