A bit like night & day really, but if the small pushrod engine has an external aluminium oil pump, on the right side of the block near the front, driven by a horizontal shaft, then chances are that it's an A series engine.

If it looks like an Austin A40 engine, if it has a cast iron head, then it's probably a J series engine.

An examination of the engine number will reveal all. These two Datsun engines have the engine letter code & the engine capacity code as a prefix to the engine number.