I very much doubt that you'll find *any* LED with a 40cd rating.
40mcd yes. I have just bought a white LED with an 8000mcd rating and 20 deg. viewing angle, there may be more powerful ones around but it will be a while before we see 40,000mcd (40cd) LED's.
You can control the light intensity by changing the current through the LED or, if you have an array, you can change the current to all LED'd. This would be preferable to switching individual LED's as then you no longer get the light spread of an array. BTW, fit a diffuser as well for a more even spread of the point source light.
Each LED should have its own dropping resistor. To control the light output vary the supply voltage after you calculated the resitor value at the max voltage.
Example (substitute specs of your own LED's):
I LED = 25mA
V supply = 12V (max)
Vf LED = 3.6V
VR = V - Vf = 12 - 3.6 = 8.4V
R = VR/ I = 8.4/.025 = 336 (use 330 Ohm resistors for each LED)
Klaus