You will need to change the truck oil pan and oil pump pickup over to a car oil pan and oil pump pickup. I believe you can use the pan and pickup off your 68 Dart's 318 (after cleaning it well) on the truck 318.

You will also need to use the left (driver side) exhaust manifold on the truck motor to clear the steering column.

You may also have to use the flex plate or flywheel out of your Dart, depending on which transmission/bellhousing you have.

If your locked up 318 is bolted to an auto transmission, you will need to pull the motor with the converter attached because you won't be able to remove the flex plate to converter bolts without turning the motor over. That could be a tight squeeze on a 68 Dart. Once the motor is out, you can reach all the flex plate to converter bolts with an open end wrench. Be prepared, the converter is a bit heavy and those 4 bolts are the only thing holding it in place.