What type of harness do you have? You've made a lot of changes without resolving the issue, so it will take some interactive troubleshooting to find the root cause.

If you have an internally regulated alternator like the 10SI (1969 with A/C) then the regulator's internal diode may be bad.

If you have an externally regulated alternator like the 10DN (most non-A/C 67-69), and you did not reinstall the regulator, do you have the regulator jumper block installed?

Is your alternator actually charging? Take a voltage reading with the engine off, then another at the same point (battery positive or horn relay) with the engine running. The voltage when off should be 12.6v or above, and running should be 14.3v or above.