One of the most frequent causes for failure can be the connection at the starter. There are 2 styles of starters that fully interchange, but have a different brush arrangement. The early style has a cover band on the housing and has the 4 brushes on the circumference of the armature. (numbered 25022 25079, among others) The later style has no cover band and the 4 brushes contact the end of the armature. (numbered 25149, etc.)
The significance is that the early style cable connection is fragile, and uses an odd size retaining nut. It is essential that you hold the inner nut while tightening the outer. The brass stud is split inside the housing, and has an aluminum strip that attaches to the brushes. If the stud, or terminal, is allowed to turn, this strip is often damaged, and voltage to the brushes greatly diminished. It is tempting to give the outer nut "one last turn" to be sure the cable is tight, but you will likely ruin the connection. The other thing that can happen is the insulator stops, well, insulating.
The later style of starter are less prone to this problem, but it's always recommended to hold the inner nut while tightening the cable. And remember, the connection is "hot" and your wrench will ground, unless you've taken the precaution to disconnect the battery first.
I have way too much familiarity with this...... waaaay too much.
Peter C