Horn issue...

I'm not sure I follow the what was checked with the ohmmeter. Sheet metal (dashborad) to column?
It does go to something 67dart273 frequently points out - an ohmmeter is only good for identifying breaks and really poor connections and wires.

The '67 cars (and probably many others) had a ground wire for the column on the upper attaching bolts.

Since replacing the relay fixed the problem temporarily, maybe what happened is there is oxidation/ corrosion on several of the connections.