1999 Dakota, 2014 Grand Caravan
You are right 1960 Valiant was the reason, lower hood-line they needed a slant 6. I'm thinking the 225 made till the late 90's?The 170 slant-6 was around from the beginning - 1961, I believe. It was stroked to 225 CID as an optional engine with more power.
Around 1971 or thereabouts, the 170 was stroked a little more to make 198 CID, for just enough power to offset what had been lost to emissions controls. The 225 remained. Eventually (1974 or 1975?) they did away with the 198, as it just had too little power for even a base engine.
Others can provide the exact years, but that's the history of it.