If we look at the production numbers for 1969, there were 503,352 1969 Chevelles built. This is all Chevelle models. SS396 production for that year supposedly was 86,307. So roughly 17% of all Chevelles built in 1969 were SS models, which means 17% came with a 396.
We can probably assume that the Fremont assembly plant built all models of Chevelles for 1969, so likely 17% of their production was SS396 models.
If we look at 100 cars on a production line, and if 17% are SS cars (determined based on actual production numbers), that comes out to one out of every 6 cars.
With the VINs being close, the date codes will naturally be close as well, so that's not necessarily additional proof, so to speak.
My conclusion: every 6th Chevelle built in 1969 was an SS396, so yes the VINs could run that close and therefore that is most likely not the original engine for your car. Of course, mistakes are always a possibility.
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1969 Chevelle SS396 L78 M22 4:10 Tuxedo Black
1970 LS3 Malibu 400 BB convertible
1970 LS3 Malibu 400 BB coupe
1970 Chevelle 300 series LS3 400 BB coupe, special order Monaco Orange
1970 Chevelle Concours Estate LS3 400 BB wagon
1970 Chevelle SS396 L78 M21 3:55 Tuxedo Black
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