I asked the same question on chevytalk an this is the answer I got.
Originally Posted by 62BillT
They made 2 Trunk Lids in '66. The Impala 2-Dr HT Lid, and all others.
In other words, they are all the same (2 and 4-Dr Sedans, 4-Dr Hardtops, Convertibles, and Caprices) except for the Impala 2-Dr HT (and SS).
The 2-Dr Impala HT takes a Shorter Lid.
Although the Trim will be different on some.
The above is what I learned a year or so back. I bought a trunk lid for my 66 Biscayne 2-door from a local expert, (he's been into B-bodies only for years). He sold me a 66 Caprice lid and explained all this to me. BTW, I got lucky & got it for $60!