Assume that the angle is at the rear of the truck bed, and that the length of the truck bed is 12'. Then:
opp side
tan theta = --------------
adj side
becomes:
8 ft
tan theta = --------- = 2/3 = approx. 0.667
12 ft
Then tan theta = 2/3, and arctan (2/3) = 0.589 radian, or 33.7 degrees.
This is less than 45 deg, so the answer is NO, the clay will not pour out easily.