Help for Changing the ATMega328's clock source to External Clock
Copy the existing entry, changing the prefix used; each entry must have a unique identifier, and the menu shows the various &lt;identifier&gt;.name values for each entry. Only change the variables that matter for the new chip, in this case the f_cpu value.

If not using a bootloader then change the maximum_size parameter to the full size of the flash on the chip, i.e. 32768, and don't forget to unprogram the BOOTRST fuse. Otherwise set the value to complement the BOOTSZ fuses and compiled bootloader size.