The adjustment is on the power steering pump. The lover pump to bracket bolt needs to be loosened, and then the upper bolt needs to be loosened. Tension the belt. CAUTION, Do not pry on reservoir or pull on filler neck. The service manual says to use a piece of bar stock that is inserted between the pump front housing and something that you can get leverage on. The picture doesn't show the leverage point. Do not over tighen the belt, or damage will happen to both the pump and the compressor. Tighten in the reverse order of bolt loosening. These bolts might be either SAE or Metric, since the 1977 Cadillac was a transition year for the bolt standard.
You should purchase the 1977 Cadillac Service Manual, because so much of that engine has specific service requirements. Without the manual, I couldn't have given you instructions.