Found the dimensions I needed for the BK12 and BF12 mounting blocks.... they are 60x43 and the bearing centre is 32.5 from the base. So just the one remaining query:

How do you guys normally accurately align the bearings and ballnut?

I'm guessing I need to allow some leeway in order to be able to adjust during setup.... Is this normally done at the ballnut or at one of the bearings and if so which bearing?