Hi, I would like to ask for help from people who have done conversions of road frames to fixed.

At a bike shop I frequent, the owner has this red steel salsa cycles frame with a 126mm rear horizontal drop out spacing. I was planning to get some IRO hubs and respace the hubs to fit the rear dropout. Now the only thing that I have a bit of problem (and inexperience) with is the BB spacing. It has a 73mm BB width (English thread I think, not much time lately to check). So what do I put there to achieve the "magic" chainline of 42mm with Sugino 75 track cranks?

Would the difference between a 73mm and 68mm BB width affect the chainline? Like a 68x113 seems to me like an effective 73x108. Of course I can only think of this when I am tired at night and back from work and my head is killing me...

I have questions but being in a country where there are almost no fixed gear riders, no velodrome or riding track and almost no track parts available is a little tough to get answers. I have to mail order in the hubs, cranks, cog, lockring and crank (so far) to build up the frame. That or get the only track gruppo available here... Dura-Ace.