I replaced my rear main on my 51 when I rebuilt the motor. As an experiment I tried removing the upper half as per the tech hint. The upper half would not budge. It was only after lowering the crank was it possible to remove the upper half. These results are different for many cases. The one thing is that if the seal gets damaged and it won't come out you then have to lower the crank.
Adjusting the shims in the rear main can help some with the leak problem. This was standard procedure in the days that these trucks were serviced.
The neoprene seal is not deemed appropriate for the early sixes. I believe this changed somewhere in the late fifties. The reason is the machining in the block cavity is not precise enough for the neoprene seal.
