Stacking aluminum lowering block?

I would not stack multiple lowering blocks. Get the right size for what you need. I suppose you could weld them together if you stacked a couple smaller ones to get the height you wanted, but I wouldn't run them stacked and not welded together.

As far as how much the block will lower the car, it will lower it the same amount as the height of the block. The block goes between the spring and the axle. So if the block is 1" tall, you will lower exactly 1" because that's how far you raised the axle with respect to the spring.