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?

It should not be necessary to change the rear springs because of the BBC.
If they are the original springs they may (probably) have sagged.
You might look for some station wagon rear springs for the back to raise it back up.

Mine were not affected by the swap. The front definitely was, but the rear did not change. Best advice would be wait and see before you spend any $