Hissing Solved! It was the booster. I went to the North Houston LKQ Pick and Pull today and grabbed a Big Booster from an 83 300SD. They have three 300sd's and one 300d. Most of their parts still there. There was also an 80s 300E in there.
Direct bolt in swap, I reused my Master on it. Also, I managed to finagle it in without disconnecting any brake lines. Only had to pull the clutch hose and cap the reservoir. I was a little worried about the clutch hose reaching the master but ended up having plenty of slack.
