You can run into trouble using Uno board def because it will think it has 1.5k more space than it actually does, and depending on where the binary starts (ie, if it starts where the uno bootloader does and just leaves BOOTRST at the same location as the nano, I think you can break the bootloader by uploading a sketch larger than the nano could accept. If it starts the bootloader where the nano bootloader starts (and leaves 1.5k open after it), then it would just fail to finish the upload, leaving a partly broken application uploaded.
The correct settings for your board are 1.6.21 of the avr board package and Nano, with the 328p (not old bootloader - old bootloader is what you get if selecting the 328p option on pre-1.6.21 board package)
Or connect it to an ISP programmer and bootload it as an uno, and thereafter call it an uno (as that's what it will be, effectively) - this will give you the extra 1.5k of available flash and let you use it.