The ECM receives signals from the CAM and CRANK signals to know when to fire the coil,does this truck have theses sensors???
I would start by checking your distributor and seeing if it is timed to the engine......number one cylinder at TDC and distributor rotor at number one spark plug.
as for the sensors....could be an air gap issue with the sensors in the new engine...........could be diffrent refrence markings on either the flywheel or cam gear or where ever the sensors are picking up their reference signal from......could be diffrent cam gear refrence marks....HAVE SEEN THIS ON NISSANS WITH TWO DIFFRENT CAM GEARS AND WRONG ONE INSTALLED ON ENGINE...... Start with the basics i guess....if youve verified that the timing marks on the engine are correct then check distributor timing....check coil to make sure its good......cap and rotor...plugs and wires all GOOD then sensor inputs to ECM...the ecm wont know when to,EXACTLY fire the coil with out the correct signals at the correct time......