i'm pretty confused about the sizing of the brackets and suspension bushes, etc not just for upper wishbones but probably lower and elsewhere also.

so...mounting brackets (by book) are sized at 48mm which gives an inside length of about 42mm,right? all sizes for bush tubes/bushes are typically given as 36mm - so that's a 6mm gap?? is that correct? if so then that means all suspension mounts need to include spacers. Elsewhere, rear suspension mounting brackets I've seen referenced tend to be 50mm (44mm internal) and 36mm tubes so an 8mm variance???

Why aren't the mounting brackets and bushes/tubes matched more closely? are these incorrect measurements or is it correct to design the use of spacers? and finally what type of spacers are used?
thanks all

Use penny washers to space out the brackets. The idea is that this gives you some scope to adjust the castor angle.