Glad you got it fixed! I ran into the exact same thing on my dad's 440-6 road runner a few years ago.

MSD ignitions need a very good ground and don't try to start the car if the battery is weak...they hate low voltage.

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