Bass River Bridge Mystery Cache

The cache is not at the posted coordinates.  Do not stop to look for it there because you will be standing in the middle of the Garden State Parkway (GSP).  The GZ is not accessible from the GSP either.  In summary - there's no reason to be on the GSP.

The new Bass River Bridge is part of the Garden State Widening program. They're increasing the roadway from 2 lanes each way, to 3 lanes each way. In addition, the new structures will now provide for full shoulders. The Bass River Bridge is essentially a cut/paste of the Mullica River Bridge just South of here. You can go to GC4184D for a summary of that structure (and get the cache while you're there). This structure is a little shorter and is supported on driven piles instead of drilled shafts.

In order to find the cache, you'll have to solve this cipher:

When you're at the GZ, there is no reason to climb on what you find there.  The caches is at +/- ground level.

Thanks to billdavidsaurus for helping me to insert the puzzle into the cache page! 

Thanks to Exit Stage Left for help with the geochecker