Lowering block question?

[LGCC10]

I need help with the size of the lowering block I should use. I will be installing 2 1/2 drop spindles with air bags in the front. In the rear I have put a large c notch and will also be using bags with the original trailing arms. How do you know what size block to use? I would like for the truck to lay as low as possible with this set up. If it makes a difference I plan on using firestone bags. 2600's in front,2500's in the rear.

Thanks for the help,

Pete
67 chevy stepside