I relined the brakes on my 1930 614 2 years ago and ran into the same problem. It has the 2 shoe Steeldraulic brakes. I had new technology racing material riveted to the shoes by a company in California that specializes in racing applications.
In addition to the cam, there is also a second post on the backing plate. I found that I had to loosen the retaining nut on the back of the post to free up clearance. I was also careful to place the correct shoe in the proper orientation. It was a very close fit at best, but the improvement in braking with the high tech lining is impressive!