273 Commando - UTG
IMHO there is nothing wrong with the 273 for the time period. Ford had the 260 and 289 and Chev had bored the 265 to 283. All good engines that got further displacement increases. And they were reliable. The SBM was generally good for 150,000 miles while the Ford and Chev were usually pretty clapped out by 100,000 miles.
The muscle car quest for power was on, so they got bored and/or stroked or put to pasture for big blocks.