Lights dimming and brightening

All of that looks to correct and fresh to even consider adding wires or upgrading the charging system.
Check the battery ground to block. Check the engine ground to firewall. Check the headlight harness ground to radiater core support.
There is a replacement voltage regulater available that looks identical to that mechanical unit on the outside but solid state on the inside.
Your original round back altenater puts out only 36 or 42 amps which isn't enough if you add lots of aftermarket stuff like sterios, etc.. but was enough when the car was new and the headlights didn't blare with engine rpm then.
This may be a fun one to fix. Good luck