Intake Manifold Assistance Needed

I ran across a add on e bay for a 2X4 intake manifold for a smallblock.
During reading the description the seller states the manifold only fits like 64 -65 273 engines only. I thought all LA engines werebasically the same and heads,intakes etc would interchange. I do not understand what the differences would be does anyone know.Also Do you know where to get a 2X4 intake that will work on a 340

Thanks

Bobby