My schematic on my 68 shows the fusible link wire goes from the top post of the starter relay to the "J" terminal in the bulkhead (yours might be a different terminal but the same method applies). Set your multimeter to DC volts, place your black lead on the battery ground terminal and the red lead on the top post of the starter relay. Should see full battery voltage there. Then take the red lead and stick it in the J terminal in the bulkhead and make contact with the metal connector in there. You should see essentially the same voltage at that point. If there's no voltage or a much lower voltage, your fusible link is blown. Ensure you are making good contact with that terminal. You can fool yourself into thinking you don't have voltage if you aren't making good contact.
Also check your fuses under the dash. Set your multimeter to ohms and place the leads across each fuse and ensure that you have continuity across each one.
If anything is blown, you need to figure out why. There's more than likely a short somewhere.