Factory RPO codes have always been 3 alphanumeric characters, with no hyphenation. The marketing department and advertising advisors are the reason that some of them got hyphenated, as in the Z-28 model conversion RPO, Z-71 suspension package on some trucks, Z-24 conversion on Cavaliers, ZR1 conversion of some Y-bodies, and various famous or infamous powerplant RPO codes, such as the early LS7, LS9, L88, LT1, LT5, etcetera.
Further, the original LT1 was available in the 1970-up model year cars:
Notice the LACK of a hyphen in the "Z28" badge, just as it should be.
That early version engine designated "LT1" by the factory production control staff was not substantially different from the original SBC of 1955. It was no more and no less a basis for the later LT1 than the original 265" SBC of 1955. The only thing they share other than dimensions is a resurrected RPO code designation. The most significant difference in the later LT1s was the cooling passages cast at the front of the block and all the supporting bolted-on parts. Even the rods on the 1992-93 LT1s were not special.
A more period correct engine for the '66 Impala or Caprice would be a 230/250" inline six, SBC 283, 327, or in rare cases, a BBC 396. The car could have probably had a Saginaw 3-speed on the column, PowerGlide/ST300, M21, or TH400. My father had a turquoise '66 Caprice, white convertible, 283/PowerGlide. I had an uncle with a black '66 BelAir 250/3-on-the-tree.