Berry ,
What yr is the truck engine ? On the EEC IV system , you will have to remove the SPOUT connector ....Its two wires that are next to the Distributor Cap and the wires have a square , Usually Grey , connector that you remove to check /adjust the timing . Place the inductive pickup from your timing light on the No.1 spark plug wire , the timing mark is NOT the big indentations on the crank pulley , the timing mark is a smaller mark that is best seen if you find it with the engine off and dab some paint across the mark . Unless the distributor was moved or some other work was some to the timing gears on this engine , its unlikely the engine will be out of time since , as mentioned in the previous post , it uses gears ...one on the crank and one on the cam . The gears dont allow the same wear and slack as a timing chain does .
Paul