Mock up all of your piping that goes in the bed(i.e. t-pipe, 90° pipes and tips) get it all centered in the how you like it and drill a small guide hole in the bed from the top. Go underneath your bed and find the hole from below. Measure a 5" diameter circle around it and see if everything clears. If it does cut the big hole, if it doesn't restart the process and make the corrections. Just make sure you remember which hole is the good one. I know this isn't what you asked for but it the only help I could offer.