[AZKen]

Thanks for coming back to report. I'll have to give you credit for all the checking using our guesses. I would say the "clunk" is a "Click". It is the solenoid electromagnet pulling the plunger in and sending the gear into engagement. BUT the electrical contact made by this action is not sufficient for a 450 amp starter load on the electrical system involved. Only works when no load on bench.
Assuming it is in fact the correct starter and you received that starter, and some counter jockey is not giving you a 305 Chevy V8 starter.............

NO MATTER what a local shop or anyone says..........put on another battery from another vehicle!!!!!!!!!!!

If no change:
You need to concentrate 100% on power and ground to the starter.
1. You need to have new large Gauge cables.
2. You need to clean battery post
3. Bare metal on ground connection to motor/starter bolt
4. I see in pics of that starter that is is black all over. I don't know if it is paint or mill scale or cast color. Nevertheless YOU MUST sand to bare metal, the face of the starter that butts to bell housing. YOU MUST sand to bare metal, the face on bell housing. Brake cleaner does nothing.
5. Assuming you did not paint the starter or bell housing. Assuming no gasket, shims or grease on faces.