GM-H stopped using the year-in-decade prefix to the chassis number around the 12th December 1958, so there are about 3 weeks' worth of 1958 FCs with an FC-xxxx-N chassis number.
Your engine number (if it's original) would suggest that it is a mid-to-late December 1958 built vehicle. See
this page for more info.
And to answer your question, the only way to be sure about the engine being the factory fitted item is if you have the original purchase papers. However, Ken's data would be able to triangulate the chassis number, body number and engine number and give you a pretty confident assessment of whether they match or not. Reply with the chassis number.
Hope that helps.
cheers
RET