If we are looking for a box safety then I wouldn't use the 58th pick on one.
We need the best player we can get whether it's a "box safety" whose primary responsibility is short-area coverage and stopping the run or a deep coverage safety. Richard likes to use a lot of man principles with one safety playing centerfield and another playing underneath. If Rapp (or another safety) is better than what we have at playing underneath while being strong in run support, I have no problem spending the 58th pick on him. Different responsibilities do not make the position any less valuable.
Now, you may be using box safety to mean that he can't play the pass, even underneath coverage, but I don't believe that is supposed to be the case. He doesn't have ideal speed to play the deep end, but I don't think that's going to be his role. He might have to play some Cover 2 at times, but that is less about speed than reading what's coming and being in the right position.
Right now, one of my main concerns on defense is not having a safety who is stout against the run but also can handle his coverage responsibilities well. Heath can cover but plays the run poorly. Iloka plays the run well, but I'm not sure about his underneath coverage.
I'm not saying I'm sold on Rapp, just that if Dallas believes he can play the run well and execute his duties in underneath coverage, then that's worth the 58 to me because it fixes one of our problem areas.