i think i see where you are trying to get clear in your mind, this may not be the absolute correct answer but here is how i tackled the process of fitting the parts you mention,......
the main point being CP1 & CP2, i,ll explain..... by positioning these parts into place & fully welding them onto BR12 & SB3 they ideally need to be totally flush at the front edge at the lean angle of the seat back , so even tho CP1 & CP2 are 3mm thickness don,t forget the actual lean angle, so yes 1.1mm is not going to create a major structural problem, i,m sure others have thought the same but if 1mm difference in the length of the chassis is the only concern then i for one would be happy to accept it.
my only suggestion is to trial fit the parts using clamps/magnets to check the whole area is "square", thats more important due to the geometry of the rear suspension.
hope that makes sense & maybe helps.
cheers
andy
