The 198 was a destroked 225 Slant Six built only 1970–1972 to meet a price-class slot below the 225. Same 30°-canted iron block as the rest of the Slant Six family — the family Chrysler launched in 1960 and kept in production through the early '00s in trucks. Single Holley 1-bbl, 8.4:1 compression, 125 horsepower gross.
1973+ the 198 was dropped and the 225 became the only Slant Six. So a 198 in a 1970–72 A-body car (Valiant, Duster, Demon) is correct; 198 in any later year is misread.
The Slant Six is famously durable — easy to work on, parts still available, runs forever on regular gas. Not a muscle engine; a transportation engine.