I just had my V4b open and the values for R342 and R343 are 25k ohm and 100k ohm. They are located at the top right corner of the sodimm socket. Voltage measured at the top of R342 is 2.5V. Measured at the bottom of R342 is 2.0V. When you have yours apart, measure voltage at the top and bottom of R342. Measure the resistance across both R342 and R343. If your measurements read the same then replacing R342 with a 50k ohm resistor would work and give you a voltage of ~1.66V. But that resistor is so tiny. An easier way would to solder a 100k ohm resistor in parallel with R343. By my calculations a 100k ohm resistor will drop the voltage the same as replacing R342 with a 50k ohm resistor. This is also easier because you don't have to remove anything, and the soldering is easier. Just solder one end of the resistor to the top of R343 and then the other end to any local ground, i.e., any of the big silver pads that the screws that hold the heatsink or radio frequence shield. this should get you the voltage you want. But ALWAYS test the voltage before you plug in the processor.
That is the only thing that you have to do to get 1.66V for your processor. But if you want to go for higher speeds that the processor is capable of, then follow Badflash's methods descrived in the Moderated Technical Section. You can also get the other resistors that you will need from him.
Happy Hacking, proceed at your own risk.