[raisedbyninjas]

You only need to die instantly in that timeline.

1. Write yourself a note to keep a jerry can in the Delorean, put it on your person.

2. Set destination time back by a week.

3. Marty gets in Delorean and Doc pushes it off of cliff.

4. Next of kin and Marty read note given to them at the morgue.

5. Plot of BttF 3 evaporates.

Edit: This is too complicated. Just visit Western Union again and send a second letter about the jerry can.

Edit 2: Wait, there should be a 2nd Delorean in 1885 after Marty arrives. So Doc's Delorean should have some fuel unless Doc was joy-riding. I'm really not thinking 4th dimensionally.