For your car you probably won't see any performance difference w/ a 7 series MSD. The bigger boxes do have more features though.

I had bad luck w/ 2 different MSD Digital 6 boxes, sent em back and put my old 6AL back on.

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