Not to suggest that the 390 is a good engine choice for a swap, but your info is just plain wrong. The FE series motors is smaller both dimensionally and weight wise than the 460. Popular sources put the FE motor at about 625-650lbs with iron heads and intake whereas the 460 series motors I've seen listed as 660-720lbs in full iron dress.
Originally the 351C was designed as a replacement for the FE block which started life as a 292/352 and later grew to 390/427. At 427ci the FE was about maxed out in size although people did later combine the biggest bore blocks and long stroke cranks to come out with as much as 468ci on that basic block. As we all know the 460 with it's bigger bore and taller deck is capable of well over 500ci without much effort. The 460 is a much bigger engine.
The FE motor would not make a good swap simply because it doesn't make good power without a lot of expensive parts. A 390 will never make the kind of power that even a 3.8L is capable of which just makes it a waste of time over-all.