Um, instead of converting all your flac files to wav and then to mp3, why don't convert one flac to wav, then the wav to mp3, then delete the wav, and move onto the next file? That way instead of needing a spare180Gb, you only need an spare 10Mb to do the encoding.

I imagine converting flac to wav would be a very quick process, so it wouldn't slow down your batch processing much. You could quite easily pipe the wav data from the flac decoder to the lame encoder to do it all in one step too.