It sounds like you are totally set on doing this, so I doubt you will listen, but...

You are making this way to hard. By the time you get a frame, deal with modifying and brazing painting and maybe replacing the fork, you will have spent more than any decent production fixed gear sells for.

You can certainly find a used fixed gear frame for less than you will spend for a modified frame.

A fixed specific frame will have a higher BB and will handle better in general for fixed riding.

If you really just want the look of an old converted road bike and money is not an issue, then go for it. Other wise, save your money and buy a fixed frame. It will have track ends and tight geo.