How low can I go

[Gus350]

I have a 68 swb
1) it has a factory 80's cross member
2) PB 1 inch forward A arms
3) factory inner fenders
4) 255/40/20 toys on boss 338 ( don'T remember the backspace)

My question is when aired out will it hit the inner tub and keep it from getting as low as it will go.
And how low will I be able to run it at ride height.

THANKS Kevin