I did a 400 in a 73 Duster years ago. I used the sb mounts, but I think I needed to trim the left side bracket to clear the oil pump. Ours was in a dirt track hobby stock car, so fit and finish was not a high priority. The SB mounts worked for what we needed.

I believe the biggest issue was the amount of variation in the body structure allowed by Chrysler at the time, If the variation was shifted one way, it worked out pretty well, but if it was shifted the other direction, it may not have worked at all. That is why you hear conflicting reports. I believe the DC K members were probably built with the desired shifted variation and sold as a performance part. Gene