Manual brake booster dimensions -bmw help!

Hey everyone,
Wonder if you can help me. I own a BMW e21 with an ATE brake master cylinder, which is similar to mk1 golf. I'm wanting to go down a size in master cylinder bore as I'm running non boosted brakes due to the booster hitting my dcoe carbs.
I'm aware that the manual brakes mk1s are 17mm. Which could be a good help considering the e21 cylinder is 20.64mm.
Could someone please help me with all dimensions of one of these 17mm cylinders?
Need specifically the bolt hole pattern, measurement between holes, and external piston measurement. Any help would be appreciated.
Cheers,
Sparkle