Swapping a 400 big block engine and a 727 transmission into a 1966 Dodge Dart 4 door can be a challenging project that requires a good understanding of automotive mechanics and a high level of skill and expertise. In order to complete this swap successfully, you will need to carefully plan and coordinate the various parts and components that are required, including motor mounts, transmission mounts, modifications to the steering and suspension, and a suitable rear end.
One important consideration when planning this swap is the selection of motor mounts that are compatible with the 400 big block engine and the Dart's chassis. You will need to find motor mounts that are designed to fit the engine and that are compatible with the Dart's frame and suspension system. There are a number of aftermarket motor mount kits available that are specifically designed for this type of engine swap, and these can be a good option to consider.
You will also need to find transmission mounts that are compatible with the 727 transmission and the Dart's chassis. Like motor mounts, there are a number of aftermarket transmission mount kits available that are designed for this type of engine swap, and these can be a good option to consider.
In addition to motor and transmission mounts, you will also need to make modifications to the steering and suspension of the Dart in order to accommodate the larger and heavier 400 big block engine. This may involve replacing or upgrading various components, such as the steering box, steering linkage, control arms, and leaf springs.
Finally, you will need to select a suitable rear end for the Dart. This will depend on the specific characteristics of the 400 big block engine and the overall performance goals for the vehicle. Some options to consider include a 8.75" or 9.75" rear end from a 1970s-era Dodge or Plymouth vehicle, or a Dana 60 rear end from a full-size truck or SUV.
Overall, it is important to carefully plan and coordinate the various parts and components that are required for this swap in order to ensure that everything fits and works together properly. If you are not confident in your ability to perform this type of work, it is a good idea to consult with a mechanic or a fabricator for assistance.