Is the head on or off.
If the head is on the truck you need to use compressed air to hold the valve "UP" Otherwise you can drop the valve into the engine. I have herd of people using a real long peace of string and putting it into the cylinder through the plug hole. Then they would turn the engine over by hand tell it make contact with valves. Then change the seal. turn the engine the other direction by hand and then pull the string back out.