Aluminum heads and 63 vs 65cc chambers

Gotcha, that makes sense. Couldn't one just use a thicker head gasket in that case? It just seems weird to me that they'd take the time ($) to make two options if there's a readily available work around. I guess it's worth it to them.
Yes, but hit up a ratio calc and run some numbers and be aware of the effects and dimensions of the chamber space area.