For lowering I wouldn't remove any leaves, take the second shortest and flip it on top of the stack. The lowers it by flattening the pack and you don't want to really student it too much as you will Loose traction. Removing a leaf will mean any load in the rear and you won't be able to carry anything lol. Draging your bum likes dog with worms . Legality who knows :O

As for the brakes don't add a booster unless you want to. nothing but hassles in these cars. Read how many People get more issues in 1200 with boosters.
1200 light enough not to need boosters or power steering.
Imho if stanza disks etc work fine with stock twin res master cyl, there must be another issue with your adjustments. I overtightened my rear brakes,one time and pedal hard as a Rock with no stopping power. Drums were engaged before fronts could. So fronts didn't engage properly.

Check your adjustments