Hey Jrd
I am personally not too familiar with the BM harness, but to answer your question about the backplate and collarplate, the collar plate goes over the backplate. I am not sure if there is an exact amount it should overlap,as mine I would say overlaps close to half - 3/4 an inch. but Depending on your attachment method, you could probably have more of less with it.
If possible, maybe get a different harness? The bigdane kits are honestly great. A lot of people have the perception that they are uncomfortable, but they really aren’t and some who have tried others say that Danes kit is far superior.
Hope I could help!