New to mow par x ford guy

Assuming both are automatics- get the transmission from the Diplomat, too. The slant 6 transmission will not fit a V8.
Since the 318 is a 1987, it may still have the Lean Burn system on it, which you will most likely want to delete- all this will involve is an earlier electronic ignition distributor with vacuum advance (which will utilize your existing Dart's electronics), and a standard earlier 318 non-feedback carb.
The Dippy's transmission may have a lockup convertor, which will need to be put on a manual switch; so grab the transmission harness too. This trans is a version of your Dart's 904, so all the linkages, speedo drive and driveshaft will bolt right up.
Edit: You will also need to find a driver's side A body exhaust manifold for the 318, unless you're planning on headers.