The 4.3L V6 in a Chevy Silverado doesn't have cylinders numbered 1 through 6 in a straight line like an inline-6. Because it's a V6, the cylinders are arranged in a "V" shape. There's no single universally agreed-upon "cylinder 1" location across all years and engine variations. The number 1 cylinder's location is determined by the engine's orientation in the vehicle, and the manufacturer's marking on the engine itself.
To find cylinder #1, you must consult a repair manual specific to your Silverado's year and engine code. The manual will have diagrams showing the cylinder numbering and firing order. Looking at the engine itself, there should be markings cast into the engine block or valve covers indicating cylinder number 1.
