I owned 3 American Motors cars in this time period: 1) 1965 Rambler Classic 550 with the 199 cubic inch engine; 2) 1968 Javelin with the 232 cubic inch engine; and 3) 1975 Pacer with the 258 cubic inch engine.
All three engines were derivatives from he same basic block and all of them ran well. As I remember, the Concord was a rebadged Hornet. IMHO, the AMC 6 cylinder engine was as good as the Chrysler slant 6 and possibly better. These engines had 7 main bearings. Nash, which merged with Hudson, had produced a seven bearing 6 cylinder overhead valve engine even before WW II for its Ambassador models. I would bet that some of this design carried over to the later 6 cylinder engines from American Motors.