66 Dart/74 la 318 starter issue

So I recently swapped a 1971 la318 out and a 1974 la318 in to my 1966 Dart. Haven’t changed any wiring, but I’m having a starter issue. When I turn the key, the relay clicks, but nothing else. When that happens, the interior lights dim, the fuel gauge drops back to E. battery is healthy, I swapped in a new relay, I have continuity from the relay to the starter, and when I hook up a remote starter switch to the terminals on the starter it starts and runs fine.

What am I missing? What should I be looking for? Is it a short somewhere drawing all the power from the starter when I turn the ignition?

Any insight or tips would be greatly appreciated! I’m all ready to go cruise, but I don’t want to have to crawl under the car to start it every time!