Mark have you read Chris write up here?
http://www.2wdrc.com/tech_bk2_axles.html
You're right you do need spacers, xx4 front bearing spacers are exactly the right width, don't think they are as thick as 4mm, more like 2.5? It's basically making up the difference in the difference between where the roll pin is drilled relativly between the B4 axle and BK2 axle.