HEI ignition module

You can never have too many grounds. Yes it's a good idea to ground the module as well as the alternator case. Braided ground straps are best.
Firewall to cyl head
module to strap mounting pt on the fire wall
alternator case to inner fender.
Battery to core support and engine.

Metal to metal contact is required at all pts, no paint
As far as the voltage drop discussion that is a bit of hot air as long as the wire is the correct size so size matters. Trunk mounted batteries come to mind with no voltage drop issues. Whats that volt meter set on - micro volts?
microvolt (plural microvolts)

1. One millionth (10-6) of a volt, abbreviated as µV.