This means that the spark plugs fire in the following order:
1. Cylinder #1
2. Cylinder #8
3. Cylinder #4
4. Cylinder #3
5. Cylinder #6
6. Cylinder #5
7. Cylinder #7
8. Cylinder #2
Remember, this is the standard firing order for a 250 cu in straight-six engine in a 1967 C10. Always double-check the specific firing order for your truck, as there could be variations based on engine modifications or specific model details.