You want lowering, please go for B8.
For springs, you have to request the workshop to find a suitable springs that match your required height. Cause B8 is shorter stroke (you are right) for lowering of 30-40mm. Height of the vehicle is handled by the height of the springs.
B6 will not be suitable as it may the piston may keep hitting the top because of your lowered springs.
Hence, I have no recommendation on the springs. However, Bilstein factory's policy now is to push full sport kit set, B12 equipped with springs already.
B6 and all above are all Mono-Tube. Now most of the new vehicles B4 are also mono-tube as the manufacturer's required.
Please do not mix mono-tube technology with the B4 or B6 codes. They are not mutually related
B4 = stands for original shocks as per factory
B6 = performance shock, harder and better than original equipped profile but at original vehicle height (no need to change springs)
B8 = performance shock, same profile as B6 but required to buy additional lowered springs
B10 = performance shock, Wedge shape, 60 front/40 rear lowering ...
B12 = performance shock, B8 comes springs in the kit set already
B14 - B16 = height adjustable and hardness settings