The only 100% way to tell what motor it is, is by measuring bore and stroke. There's been many 390s and 428s that have been "cross-bolted" after the fact, so a bottom picture doesn't guarantee anything either. There are also non-cross bolted factory 427s too. Depending how the builder wanted things to look, screw-in freeze plugs is also an indicator of a 427...
If ya gotta know, pull the
oil pan and measure bore and stroke.