1. There were some rifles that used the cam over spring system, but most used some sort of a latch, which is a much better system. For the cam system to be secure you must fabricate a much stronger spring than is supplied in the kits, and then that makes for difficulty in opening. There are way better methods of securing your patchbox lids. Do a little research for the style rifle you are building and build an appropriate latch. You will be happier with the result rather than trying the short cut.
2. As a general rule the box lid overlaps a recess in the buttplate and the sideplates are fit to the inside edge of the buttplate. There are exceptions, but they are few. Again, do some research. The ALR virtual museum is an excellent source.