1966 B- and C-body with 318 or with 225 (police/taxi) is correct. This transmission has the A/LA V8 bellhousing. When used behind the 225, an adaptor package was used consisting of a bellhousing adaptor plate and a crankshaft to flexplate spacer (to account for the thickness of the bellhousing adaptor plate). The starter was in the low-mount (V8) location, bolted directly to the bellhousing -- the adaptor plate is cut out in this location to clear the starter. This same setup was used in all heavy-duty slant-6 automatic applications (police/taxi cars plus trucks and vans) until the 1968 advent of the A727RG transmission with the \6 bellhousing pattern.
This trans will work behind a '74 small block, but you must install the 1/8" annular spacer ring to fill the space between the '66-down small torque converter nose and the '67-up large crankshaft counterbore. You can get it from Pat "Torqueflite Patty" Blais, among other sources.