It may be as simple as a new angle drive, or it could entail a new speedo as well. IIRC, there were 4 different speedos used in B's, each with different turn-per-mile ratios. It may be just as easy, if not easier, to get a ratio adapter made for this specific installation and install that in-line with your current cable and speedo. Here are some starting points for that:
http://www.speedometershop.com/ratio.html
http://www.gaugeguys.com/ratioadapt.htm
http://www.a1electric.com/speedometer.htm
These guys come highly recommended by the LBC community for speedo repair in general: http://www.paspeedo.com/
Cheers!
R.