it sounds like your throttle linkage is not set up correctly. either the servo is set wrong as stated before, or the linkage has taken a hit and isnt set correctly. you need to adjust it without starting the car. turn on the truck and the radio, but do not start it. have a look at the throttle linkage. you are on the right track by knowing that the slider has to close in order to stop the car. this slider is the carb. you may have accidentally turned one of the knobs on your radio and changed the servo center setting. look at the radio and try the knobs. one at a time, and be sure if what you turn doesn't do what you want that you put it back where it was and go to the next. the knobs should have a hash mark or an arrow or something showing where the center is, so it should be easy to see which one has been moved.
if all are centered, your servo horn may be stripped and has jumped the teeth on the spline drive and needs to be replaced.
check out the radio settings and get back with us.
EDIT: as rolex stated it may be the idle screw. when you move the trigger up to apply the brakes, does the engine slow down? (less RPM) or does it stay high, but the truck stops?