We are talking about the 70's right?
Your right to a degree, they where machined together, some parts might not line up exactly from one match line to the next. I've never quite believed this unless its the transmission cases. Thats where things get wierd, lots of things connect and mate together. As far as the top end you can make that work swapping parts. In this case the only two critical areas are at the two cam bearing locations.
If your building a restoration bike for a keep it 100% by all means you probably wouldnt want to use this mismatch part because it upsets you deeply inside for some weird reason.
If your building some bike out of a pile of parts.... by all means, build it.