Greg, I would tend to agree with you expect for one detail. Casting numbers only refer to the casting and do not reflect any post machining variations that may be done as a difference between DOHC and SOHC. It may be something as trivial as a mouting boss that is tapped in one version and not the other. I don't think there are any differences and apparently none that matter. But from a strictly legal sens it is hard to show proof and if the part numbers stamped don't match the motor wouldn't be legal. Also you have to keep in mind things must be interchanged as an assembly. From a practical standpoint, I don't think it matters but I do think anybody doing this should be aware of the possible rules implications.