This, again?
Bearing size (total bearing area) absolutely is the point; the crank is supported well more than adequately, and it's more than rigid enough for 4 vs 7 not to matter. Time has proven it; the Slant-6 bottom end routinely holds up for at least as long as the other-brand inline-6 engines marketed at the same time, and often longer—even the post-mid-'76 forged-crank ones with the smaller bearings. Even with extensive power upgrades.
This same baseless whinge—
Eeeeeee, it only has four main bearings instead of seven, eeeeee!—was blathered in the 1960s by lazy, sloppy auto journalist Jan Norbye. He seemed never to miss an opportunity to cluck his tongue and scold the Slant-6 for having
only four main bearings. He bìtсhed about this nonissue again and again and again. We see it here in this 1966 Popular Science article comparing the Valiant with the junk the other big three were peddling; lookit how he pretends to be astonished that the Slant-6 is so smooth and quiet despite having (tsk!) only (tsk!) four (tsk!) main bearings (tsk!):
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When he wrote this the first Slant-6s were only 6 years old, so their durability wasn't yet the matter of legend it is now, almost 60 years later. But I still don't give him a pass; he was given to spouting bozo opinions (“today’s drivers prefer warning lights rather than gauges”) and incorrect, made-up factoids ("The Torqueflite lacks water cooling"), then snappishly defending them when called out in letters to the editor saying he was full of beans. That's the pile of sewage this only-four-main-bearings-it's-a-wonder-the-Slant-6-runs-at-all idea belongs in.
Thiz
another example of latching onto one little slice of theory, which might be perfectly valid, and making like it's the only factor in whatever is under discussion—ignoring other factors that can easily overwhelm and nullify the one latched onto.