Hook the tach up to the MSD box.
The MSD's power wire should go directly to the positive battery terminal. I hooked mine to the big post on the starter relay.
MSD's are sensitive to low voltage...that's why they say to hook it directly to the battery.

CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax