465s are 75mph units, the 377s are 100mph units. I'm going to guess that a 465 will struggle to meet 377 timings. Also aren't the 465's 3+2 'inner suburban' layout i.e. no tables etc, whereas the 377s are 2+2 'outer suburban' layout ?
I don’t think any of this is that much of an issue. 465s have worked far into Kent in the past, and indeed I think still do at times. It wouldn’t be too much of an issue to reconfigure the interior as was done with the Wealden units.
Lack of SDO was mentioned on here as being an issue for the Maidstone East line though, that issue would certainly require a solution to be found.
It does seem like events may have overtaken though - store some 465s temporarily to make room for the 707s, then bring them back to release the 376s.
If the 379s were added to the mix, most likely by replacing the GN 387s then Southern would probably have enough electrostars to be able to ditch the 313 and 455 fleets. The only snag with this is what happens with the GN peak extras, if they return to their pre-2020 levels then something else would be needed.