This is what you will need to do....

1. Take everything off of the mustang block and use just the block, heads, and harmonic balancer.

2. Use your intake, valve covers, oil pan, pickup tube, and everything else.

3. Swap the distributer gear from the mustang and install it in the truck distributer.

4. Hook everything else up they way to took it apart.

5. Swap the spark plug wires around to accomidate to your new firing order.

6. Do a Mass Air Conversion.

7. Drive and enjoy.

You can do the swap without a Mass Air Conversion but it will probably not be much different from your current setup. You may even get worse gas mileage. I have done it both ways and the MAC is a much better way to go.

There is no need to swap fuel injector firing order.