If you want a square taper BB you can't beat Phil Wood. Last time I looked they had a huge range of choices of widths and tapers.

Assuming it's 68 British thread any of the modern cup BB's will screw in. There's a wide variety of compact cranks that work with exo style BB's, and they are a snap to shim, especially if you use a pinch bolt type crank (Dura Ace). I've built several old steel frames like this.