How do I tell by the heater core if the car came with a big or small block?

My understanding was the small blocks had a longer heater case in the engine compartment and the two holes for the core to come threw where at the far edge of the case. The big block heater case had the two holes for the heater core coming threw about 3/4 of the way.

If that is clear as mud. The small blocks would be at the outer edge towards the engine, the big block about 3/4 of the way to the outer edge towards the engine.

I think there where some differences for A/C or non AC?

Thanks, for the info. I have along way to go,but I'll find the history on my baby someday.