198ci

It goes back to the initial introduction of the /6. Originally the 225 was for the larger cars while the 170 was intended for what became the A-body. The 170 was about sipping fuel while delivering the car to its destination. FWIW, FoMoCo offered a 170 in the Falcon and Rustang. As engines lost power due to emissions requirements, the 170 became a performance liability which led to the development of the 198. The 170 used a shorter block, while the 225 and 198 used the same block yielding some cost savings at a time when Mopar needed it.
The 198 uses the longest connecting rod of the 3 Mopar /6 engines. To over-simplify the modification, installing the 198 rod in a 225 raises the piston in the bore raising compression ratio. Go to www.slantsix.org and do a search on long rod 225 for details. Since the 198 was made for such a short period of time, not many rods are out there.