Something odd I noticed before doing this fix....the insulation on the grounds (up next to the fuel pump bracket) looked to be heat damaged. That and the slow turning over prompted me to look closely at the starter motor and associated wiring.
What I found was one small wire strand had not been properly attached to the brushes and was actually touching the starter housing, which explains the ground insulation damage.
It left the factory that way in '85.
So my 'fix' involved new brushes, new ground wires AND the addition of the new, heavier ground.