Rear Axle Travel

[Doug19]

So I’m about ready to lower the back of my 66 and put in a custom notch. Now I’d like to make the notch as small as possible to prevent having to raise the bed floor a lot. How much travel does the rear axle need? How much space do I need between the top of the axle to the bump stop or frame? This is assuming I don't ever plan on loading anything of significant weight in the back of it and will definitely never be towing anything with it. This just will be just driven on nice sunny days.