sbc suspension with bbc?

I have the standard sbc springs all around. But when i drop my 402 in, do i necessarily have to change my rear springs to accommodate the change to bbc? I know the front would drop down because of the weight. What would be the effects of keeping them in the rear?

I'm also putting in a new 10bolt posi under it, so would it just be a good idea to go ahead and put new springs anyway??

what springs?