First thing to ask yourself ... Is why do you want hood vents. ???
Q/. Because they look cool ??
A/. Any vent type will do ... they dont even have to have holes in them.
Q/. Because everybody else has them ??
A/. A scene & quote from the monty python film - "life of brian" springs to mind ....
"
you are all individuals " .... and one voice in the crowd says "
I'm not " ....
Q/. Because of high underbonnet temps ??
A/. Diagnose the cause of the high underbonnet temps.
Research correct vent placement positions and the effects of incorrect placement.
This is good general info about heat in engine bays.
http://www.oman4x4.com/hoodvents.htm
The only thing not explained ... is that the
lesser heat build up at the centre, rear area of the engine bay ... is because heat is carried away down the tranny tunnel .... ( XJ's have a tranny tunnel too ...

)
Vents over the top of the engine only benefits at low speeds.
Vents in the rear corners of the hood provides venting at high & low speeds.
Quite an interesting subject is hood venting and airflow ... and the imagined and proven benefits.
Generally its over-shadowed by internet myth and fancy advertising claims from businesses wanting to sell bits of tin & plastic with holes cut in them ....
:cheers: