Bad Starter or Starter Solenoid?

The return current flow is thru the case of the starter (thru transmission). If testing it on the ground, connect the black jumper cable to the case ear. If you touch the red jumper to the stud right on the motor (under a rubber boot I recall), the motor should spin if it is good. Of course, hold the starter down with your foot. If you connect the red jumper to the bigger stud on the solenoid (normal wiring in car), then apply 12 V to the smaller stud (normally 12 awg brn wire to starter relay), it should actuate the solenoid (throw pinion gear outward) and the starter motor (if internal contacts are good).

Your starter could be fine and the electrical connection to the transmission bad. Aluminum is notorious for that. Did you sand the surfaces first and apply di-electric grease? I never install a starter without checking it first, even on a slant six where it is a simple 5 min job.