273 or 318 ?

Plus don't forget, the in-coming 1970 A-bodies were some 300 or more pounds heavier than the out-going 1969 models.
From 170 to 198 is an increase of ~14%, about equal to the increase in weight from, say 2700 to 3150. I don't think they had much of a choice; if the 170s had made it into the newer heavier cars, how long would it have taken before Chrysler would begin losing sales? I mean 3150/170 is an abysmal ratio of 18.5 pounds per cubic inch.