Or you could just get the entire timer box from a '78 or later 300D. Run an eight gauge or larger wire from the battery positive to the timer feed, connect a wire to the ignition circuit and be done with it. Connect the leads from the timer to the individual plugs for a 300D and tape up or remove the extra wire for a 240D.

You could even get creative and run a wire for a light in the dash for the "glow" cycle. If I remember correctly there is even a wire to connect to the temp sender to disable the timer when the engine reaches operational temperature but it's not really required.

Use a 50 amp fuse/circuit breaker on the main power feed at the battery connection.