In practice there is a marked difference between these two types of engines?

The internals, no. AFAIK, the only real difference in the "hard parts" is that the GC motor's heads may have smog pump holes in the exhaust ports--but not all of them do.

The exhaust is very different; the GC one is much worse.

The EFI is slightly different.

There may be other differences but they aren't coming to mind right now.

--DD

Both GA and GC have oil pressure relief valves under the oil cooler, but only the GA case has the oil pressure control valve in the main oil gallery under cylinder #1.