Hi,
IF you have start to remove the back of the box then can be easier just to have your speedo re-calibrated, that's what I done, over twenty years ago now when I fitted a 3.08:1 CWP and been fine, should get it done again really what with the 2.84:1 living back there.
Used these gents -
http://www.speedycables.com/
Cheers John


LIVE LIFE A QUARTER OF A MILE AT A TIME!
1976 Speke FHC BEAUTY FITTED WITH OVERDRIVE GEARBOX
1979 3.5 FHC CURRENTLY GARDEN ART !
1982 2.0 DHC NOW A 4.6, BUILT NOT BROUGHT !!!!