I ended up mounting the fence post sleeve near the front due to the bunk room slide by the dump, and the front slide/ gray tank plumbing. So the fence post sleeves were moved near the front. Pics in my mods link in my signature.
Worst case, instead of mounting side to side, maybe mount it front to rear next to the frame. Maybe you could use the uni-strut with clamps like I did. I went this route because the front sleeve has 4-6' pvc poles for a "tarp tent", middle has a GPL 7' ladder, last has the sewer hose in it with a couple u shaped gutter pieces for support. The ladder and pvc poles have a little weight to them, so wanted the sleeves mounted securely!
In the bumper I have the hot dog forks on one side, fire tongs and poker on the other side. I cut a 4" u shaped gutter so it fits in the bumper in case I can't reach something, just slide it out.