'fitting' is usually 'montage' (cf. tyres, windscreens, etc.) — though it does depend a bit on exactly WHAT it is that is being fitted?

--------------------------------------------------
Note added at 40 minutes (2015-05-14 15:13:10 GMT)
--------------------------------------------------

As has been pointed out by Chakib and Sylvie, 'fitting' in En is ambiguous, since it can refer to both 'ajustage' and 'montage'.

However, I think (in the absence of more context) that in an industrial situation, it is more likely to be a line for fitting in the sense of assembly — fitting certain components together to make a whole unit — rather than in the other sense, which as Sylvie points out is likely to take place at a 'poste' rather than having a whole line devoted to it. As ever, proper context would probably make this abundantly clear!

Do also note that in many situations involving some kind of 'production line', the term 'line' is often rendered by 'chaîne' rather than the more obvious 'ligne' — Asker may wish to bear this in mind.