Ok, you said that the 4.3 l engine that was in it went bad,,,why did the engine go bad ?
Did the lights, turn signals etc. work before the engine was replaced ?
Did you replace the " bad " 4.3l engine with an identical 4.3l engine ?
Was the replacement engine a new or rebuilt engine ?
Did you swap some of the old parts from the bad 4.3l engine to use on the replacment engine ?
You say that you know that the starter is good,,how do you know if the starter is good ?,,did you get the starter checked ?
Who is " He " ?
Who ever replaced the engine forgot to hook up all of the wires or may have damaged an electrical connection some where.
The first ground wire comes from the battery negative terminal to a body ground near the headlamp.
Another ground point comes from the front section frame rail to a front body mount area.
There is also another ground on some trucks fom the battery ground to the alternator mount hanger.