Hi
The hardware of RT-N66R is the same that RT-N66U
I use this:
K26RT-N – MIPSR2 – special builds for E4200, RT-N10U, RT-N12B1/C1, RT-N15U, RT-N53, RT-N66U, WNR3500Lv2 and newer Linksys E-series routers in Tomato Shibby, now is in the version 132.
tomato-K26USB-1.28.RT-N5x-MIPSR2-132-AIO-64K.zip
or you can use advanced Tomato
K26USB-1.28.AT-RT-N5x-MIPSR2-3.0-132-AIO-64K.trx
There are many tutorial in youtube and google, take care… read, read, read and understand after flash Tomato…
Best Regards
JulianoCWB