That hump to the left of the hole is because of the bumper shock absorber.  The area could be patched or use my spare trunk panel and eliminate the shock and then back-date the bumpers to chrome.  I have chrome bumpers here.  Back date of bumpers requires removing tabs on outer edges of front and rear bumpers and then drilling two holes per side.  The center anti-sag tab needs to be fabricated or cut-out of my donor car for the rear.  The front requires a bit more cutting to remove the edge that holds up the 5mph bumper edge, but it is an angle cutoff wheel job and paint only.  

 

The BIG issue is that the rear trunk panel is not available for a 75-76 car.  You would need to fabricate, modify, hammer-out, weld-in the humps that are over the shock.  I back dated the bumpers to deal with the NLA panel issue.  However, that changed the trunk height, the floor-pan mounting position and ultimately caused more work to deal with the bumper retrofit lower bolts.  I had to cut a hole in the bumper shock-absorber mounting plate under the car to make a way to get to the lower bolt (see the Back Dated Bumpers page).

 

See Also: http://www.914world.com/bbs2/index.php?showtopic=289586&st=60

 

Cleaned-up with a new back-half panel.  Note the four screws on the rear panel that hold the heat shield.  Some 914's were screwed in and then they welded the hea shield at some point.  This change allowed me to modify the trunk panel for the bumper back-dating and I was able to paint the heat shield with high-temp reflective paint.  No rattles, no issue noted.

 

 

Cleaned-up a bit before welding in the trunk panel.

 

Image below is from the 74'.  The arrow shows the "center" of the rear (under the lock) and a small tab with a slot is welded there to hold the bumper rubber.

 

 

Center of trunk , below lock area.  Letting water out is not such a bad idea, but this is a bit excessive.

 

And more:

 

Driver's side.  Mirror of shock mount hump for 5mph bumpers.

 

 

A view from the back of the car with the bumper removed showing the rust-through on the right side only (hidden by bumper):