Here are a few more pix to help illustrate the lip on the 'bumper'. In its OE form, the lip -- looked at from end view -- is actually beveled... lip on the top, no lip on the bottom.
When coming up with fix for the disintegrated/nonexistent lips on my bumpers, I ignored that little detail. My replacement lips were cut as strips from a sheet of 1/8", closed-cell foam. After a trial fit, I cut them to length and then glued them into each bumper. Not all that pretty, but functional (and completely out of sight after everything's assembled).
I should mention that attention to detail will pay off in this part of your restoration. More specifically, lack of attention to detail will probably result in exhaust leaks into the cabin once you get the car out on the road again (this part of the early Zs' design was, to be charitable, not very successful -- too many parts, hard to assemble, didn't work that well). As others have mentioned, several of the pieces tend to warp with age. In the case of my car, the plenum moldings had both caved in at the top, meaning that they weren't going to seal well against bumpers without a little help. I just cut a piece of plastic to the right length and glued it into the plenum as a wedge so that the plenum flange became reasonably flat again.
Another place to watch out for warping is the hatch trim panel. For my car, both ends were dished by a quarter inch or more (clearly visible in this photo). This panel has to achieve an airtight seal against the hatch sheet metal. Otherwise, the ventilation ductwork, with its one-way airflow control flaps, gets bypassed and air/exhaust gets sucked into the cabin by way of the hatch cavity*. Here, once again, I resorted to my 'squishy' sill wall foam tape to make a new gasket. Even that wasn't enough to overcome the warpage in the trim panel. I had to create some new holes on the inner hatch sheet metal to accept a pair of additional plastic trim plugs on each side of the panel. In hindsight, I probably should have spent the bucks for a new trim panel.
All of this just underscores the fact that the entire airpath from the ventilator grill to cabin entry has to be airtight and the one-way airflow control flaps have to be in good condition. If your air control flaps have settled into a permanent droop (mine had), try turning them over before reinstalling.