If a '68 or later slant-6 is used in front of a '67 or earlier automatic transmission, you must install a spacer ring to adapt the later large crank rear flange counterbore to the early small torque converter nose, or the transmission front pump bushing and seal will wear rapidly and there will be vibration problems.
To install the '75 engine into a '66 Dart, regardless of transmission selection, the '66 oil pan and pickup must be swapped onto the '75 engine, and the '66 engine mounts must be used.
No substantial transmission adaptation is necessary if the '75 transmission is used with the '75 engine; the driveshaft, '66 trans mount, '66 shift and kickdown linkage, and '66 speedo cable will all go right onto the '75 trans without a problem. The only point that will need addressing is the neutral safety switch, which is a single-pole switch in '66 (neutral safety only) but a 2-pole switch starting in '69 (neutral safety + reversing lamps). The '75 switch can be kept and just its centre pin connected to the one and only wire that will be present at this location on a '66 car.
