The firing order for a Ford 351 Windsor is always 1-5-4-2-6-3-7-8.
To find the correct timing for *your* engine:
1. Identify your distributor: Look at the distributor itself. It may have markings indicating the manufacturer (e.g., Ford, Motorcraft, or an aftermarket brand).
2. Consult a repair manual: A repair manual specific to your 1988 F250 with the 351 Windsor engine is the best source. This manual will have diagrams showing the correct rotor position relative to the cap terminals for the given distributor. The manual will also give the initial timing setting (usually expressed in degrees BTDC - Before Top Dead Center). You will need to check the timing with a timing light.
3. Use an online resource: Some online automotive databases might have information on distributor cap wiring for a 1988 F250 351W, but these are less reliable than a dedicated repair manual. Be sure to specify the exact year, engine code (if you have it), and any other relevant details.
Without the distributor information, providing a precise timing order is impossible. Attempting to guess could severely damage your engine. Always rely on a reliable source of information specific to your vehicle.