I just posted this in the vintage and classic but figure i'd repost here as well.

So i have this really old mercier with a cottered crank. The bb is shot and i came across a french campy bb that i'm probably going to put on. I want to put on a square tapered spindle which shouldn't be a problem but i was wondering how long a spindle i need? I'm also going to make this a fixed gear bike if that makes any difference.

depends on your cranks, 110 or 109.5 for track cranks usually, 115mm is usually pretty close for mtb cranks, some others have specific spindle lengths

120mm dropouts with usual track hub need a 42mm chainline I do believe

I ran into this very problem when putting a cotterless spindle in my old French BB. In the end, I just got three different length spindles from the LBS (used, of course, so they only cost $10/each), took 'em all home, found which one gave me the straightest chainline with the cranks I wanted to use, and returned the other two. (I suppose I could have just brought the bike to the shop, but it's easier to carry three spindles than one frame.)