You need to confirm that the existing engine is the original, there is far from any guarantee. As you found, the early cars didn't have engine numbers to verify this beyond doubt. What is generally regarded as acceptable evidence of originality is a casting date that is reasonably close to the production date of the car.
I don't think there is any difference between the blocks in a garden variety C code and an A code but I think if it was me I'd be keeping an original block if the casting date lines up with the production date, even if you don't have the original A code manifold, Autolite 4100 etc. If you do have all original external A code bits you've replaced then this course of action is a no brainer. By all means put a stroker in it but store the original away.