I don't think there is any hard and fast rule you can apply. I suspect there were customers who may have had their 396 warmed over with a cam, intake carb and such. There are also cars which probably had ther short blocks switched out for 427s. I also think there were pleny of L88 conversions.
I read a comment about this engine pic still having exhaust manifolds on it. Probably isn't a pic of a Nova. Also noted is the fact that the 396 option was not available till after this article was written. I guess if that is the case, any early 68 Chevy II 427 Motion cars probably were full engine swaps from a small block car.
I don't know how long it took form ordering a car to delivery. Don't think Baldwin/Joel would have calculated to take orders and wait for the 396 option to become available. From a salesman's perspective I would either "up-sell" the customer into a BB Camaro or "up-sell" into converting a SB Chevy II.
All these points changed each year, so again- no hard and fast rules.