When the Trans-Am rules from the SCCA came along, it was intended to level the playing field, with all cars not being able to weigh less than 3200 lbs, and all engines at a maximum capacity of 5000 ccs. It wasnt till the E-body cars that Mopar got back in to it, although somewhat half heartedly.
In 1967, Ron Grable ran a specially prepared 67 Dart, which was really a remnant of any interest by the factory. This car like the 66 cars was specially made, with a number of chassis tricks. Unfortunately in order to build a 5000cc engine, it had to be based on the 318, with an early pre LA 318 truck crank which had a shorter stroke. The car also had to run a custom flywheel, as the crank had a different offset, and rear seal from the later engine. Grables effort didnt have factory sponsorship, or the money necessary to make the custom engine parts that would have been necessary. Grables '67 car has been more or less restored, and is running the vintage circuit, although the current car is running a much more powerful and modern engine based on a 340; an engine that the original car never saw.