I have a 99' SE and from other's mods done here and checking in the junkyards i learn some are interchangable and some parts are not. Heres what i've found.
Door mirrors, Clusters,rims,shifters,shifter knob,seats,steering wheel,radio,ATC,console,interior door handles, window&lock switches,OTIS & EVIC and motor parts are interchangable.
Brezel covers (ie. shifter brezel, dash and the peice around the radio. LHS & 300M are the same ) , door panels ( the front are but the back ones have to modded ) ,tail light , head lights and doors are not interchangable.
Now the great one YBNormal has done the complete front end change one his trepid to a 300m front clip. So that can be done but you have to change the whole front clip.
I sure there's more, that just what i can think of off the top of my head. And if i'm mistaken i'm sure i'll be corrected and sent back to the corner.
and not all things that are interchangable are plug and plaly. IE, a 300m cluster will be a cluster for autostick. While one out of a concorde or LHS will not be autostick. So you'll need more then just the cluster form a 300m if you do not have auto stick.