In case this helps...
There are two 65-66 decklids on the market currently, the one from Golden Legion, and the newer-tooling/thicker one from Musclecar GT (which I'm fairly certain where Dynacorn buys theirs from).
We carry both.
I went back with a measuring tape, and the MGT decklid came in at 52 5/16" wide (measuring in the same place as the OP).
The GL decklid came in at a gnat's hair over 52 3/8".
I measured my 1,300 original mile original-paint coupe in the showroom, and it came in at just-over 52 3/8", you could attribute the difference between it and the GL decklid to paint thickness.
And if the OP has a couple of repaints on his original lid, that could explain the extra few gnat hairs on his tape.
But it does appear that the MGT lid is a touch narrow..
What I was not impressed with is the care with which the outer skin was applied to the inner frame on the MGT lid, or the craftsmanship on the tail-end of the lid. Where the body creases are, it's not terribly crisp, and not terribly straight. The body-crease seems to meander a bit. And lots of grinding work under the primer on the RH side.
For the extra dough, you might get O.E. thickness and heft, but the lighter GL lid has the better side to side dimension, and better/crisper/truer stamping of the outer skin, and better/cleaner installation of the outer skin.
Of course, installing each of them is the REAL litmus test, and I don't have an available coupe to play with. I can't play with that virgin low-mile unit in the showroom obviously..
To the OP, I'd be willing to slide you a killer deal in return for your test-install findings on the GL lid, if you're so inclined. PM me if interested.
Water drains...... And I thought I'd seen it all....