C-body mounts into A- body????

Was doing a mock up in my 71 valiant that was 318 auto. Had a 74 400 block out of a C-body that had spool mounts on it. So I set it in the car and the mounts lined up and it went right in place?! So my ? Is why does this work or not work. There has got to be something I'm missing here?? It can't be that easy.

Also my oil pan(aftermarket) clears everything and oil pump clears the K-member barely.