Larger Negative battery cable should run to the engine, often one of the starter motor bolts, or to a bolt on the block or motor mount(engine side)
This larger cable is for starter motor AND alternator ground, those are the big AMP devices, stater 60-75amp, alternator 80-120amp
Smaller negative cables on battery should run to Rad Support(head lights) and inner fender for relay grounds.
There should be a Ground strap/wire from the back of the head to the Firewall, this is the main ground for cab electrics
This is often left off after engine work, put it back on or add one
Then there will be a Frame ground strap/wire, often on motor mount, but from engine to Frame(tail lights and fuel pump)
Can't have too many Grounds
Remember, all metal parts on the vehicle were painted BEFORE they were assembled at the factory, and many have rubber washers on bolts that connect them together, so never "assume" a good ground because it is a metal part of a vehicle
As far as starter motor not turning and getting a "click, click, click", that does mean starter is getting less than 60amps, lights and such need maybe 5-10amps at most, so no comparison with the 60-75 amps needed by starter motors.
Lights can work fine, doesn't mean much
So you are on the right track
Battery Positive cable needs to pass 60+ amps to starter motor, and Battery's negative cable needs to carry that same 60+ amps to complete the Circuit.
Check both for good clean bare metal connections