Lowest power possible in Arduino
The issues regarding efficiency of power are:


The CPU speed
The Voltage 
The Power Regulator (not supply)
The Peripherals


Address each one of these and don't over look any of them. Or any sub component. But note that some are more impacting than others.


while it is possible to lower the speed, It is more practical to just sleep. As you have indicated you are going to do.
Running at 3V instead of 5V is a big savings. Note that the 328 can run as low as 1.8V
DON'T use Linear Regulators. Most all Arduino's have a 7-12V to 5V(or 3.3V) Linear Regulator. This just burns the difference between the supply (aka battery) and the VCC, used.
pay attention to the peripherals. Likely this means getting rid of not needed devices. Example the USB to Serial chip that exists on the Nano. What is the lowest voltage can the radio run at? Must it run at a regulated voltage. Use the largest possible resistor values. Such as on pull ups. A 500K pull up will draw less current than a 10K. Where if too big any capacitance may slow it down. Which is likely acceptable.


One of the biggest savings is running directly off of the Battery without a Linear Regulator. Where the ATmega328P can run from 3V down to 1.6V. It would be recommend to use the Brown Out detector.

Also note that ALL batteries have a self discharge rate.  This will determine the maximum possible life of the battery, then minus your devices consumption.