Been down this road with a customers 71 duster that came in with sbp axles that were converted to bbp. The drums used were bbp that don't work with sbp alxes. I machined new sbp drums on my mill for bbp and the 10 x 1 3/4 brakes and everything was good. We have since changed the car to moser bbp...
Who made the converter? I had a ACC converter do the same thing. Called ACC and had to send it back. They told me it was a defective converter, but they didn't tell me what was wrong. They sent me a another one and all was good.