Building up first SS, need some advice on BB + crank dimensions

2. I am 6ft4, which crank length should I go for? Is it very important to get the correct length, even if it's just 2.5mm different?

Everyone's built with different proportions, but I'm 6ft 4 plus and find 175 cranks suit me well. I wouldn't say 2.5 mm makes enough difference to notice, but 5 mm does. Since it's a SS bike you're building (rather than fixed) 175 mm cranks won't give you any problems cornering as you can freewheel if necessary. In any case, 175 mm cranks don't give me problems cornering on my fixed as (a) I use fairly slim pedals with good ground clearance, and (b) I don't go crazy around corners anyway.

For your height, I don't think you'd go far wrong with 175 mm cranks.