Really there are a couple parts to your answer.
1) by the time you're adding a caddy 500 into a front clip you are going to want to rebuild the front end anyways - factory brakes will be insufficient, spring won't be strong enough, and since it's apart anyways, bushings and ball joints only make sense.
You should be able to find a cheap gbody that is rotted in the rear frame rails as a donor.
2) depending on your driving style, you may want a close ratio steering box. You do NOT need a monte ss/442/ttype/gn to get one, just the right option code from a donor ad any of the cars were able to be ordered with a performance suspension.
3) each of the various gbody cars had their own unique frame braces that other models didn't get. I'd point to the cutlass and grand prix for triangulation braces to help stiffen things up. If you're on a budget, f body larger front sway bar is a good choice as well.