Correct size bottom bracket?

Barrington12 Posts: 358
edited June 2014 in Workshop
Hi, wonder if anyone can give me some advice? Hope this is not a dumb question, and I think I might know the answer already but would appreciate some knowledgable input....
I have a Specialized Langster frame which I will be building up into a single speed/fixie and I am looking at chainsets and deciding which to buy. I have seen a Stronglight chainset which looks great and it says on the details 'ideally suited to bottom bracket 107mm conical square for a 42mm chainline'
The Langster details I found on the archive page on the Specialized site state that the Langster was originally fitted with a 103mm bb.
So my question is do I buy the bottom bracket to suit the chainset, ie 107mm, or do I need a 103mm because that's what the frame originally was fitted with?