How to figure out which BB to buy
 

If I have a custom frame and I'm nearing completion.. How do you figure out which BB to buy size wise so I'll get a good chain line?

chainline, assuming a properly aligned frame, is only contingent upon your hub, cog, bottom bracket, and cranks.

the only frame-related issues i've really run into are when the chainring/cranks bottom out on the seatstays. you tend to only really see this on frames wide rear spacing or when trying to pull in the chainline of certain road cranks. i'm assuming that your frame is designed with track/fixed in mind, so it should be fine.

Typically a 110 BB will work if you are using all track equiptment.

a cartridge-style bottom bracket for which neither "cup" has a stop flange (a phil wood, for example) will allow you to move the spindle to the left or right a few millimeters either way. but yeah, like ira says, for 120 mm rear spacing, 110 mm is a good bet.