Originally Posted by 1993 SLE
the SSE/SSEi grille will fit on the SE/SLE but the SE/SLE grille WILL NOT fit on the SSE/SSEI
Right; please let'* not abuse the language... I don't think it'* accurate to say that they're really interchangeable. They're not the same shape, and each is intended to mate up with a different front bumper cover.
The fact that one mismatch will cause the parts to collide (SE grille over SSE cover), and the other mismatch will cause a larger-than-normal gap (SSE grille over SE cover) should be a tipoff that they're really not intended to go together any old way. The SE grille goes with the SE bumper cover, and the SSE/SSEi grille goes with the SSE/SSEi bumper cover.
If you want to go back a few years, it was possible to bolt a 1968 LeMans/Tempest nose onto a 1969 car, and vice versa, and that was a completely painless transformation; anyone who didn't notice what kind of taillight design it had wouldn't notice the change at all.
(If you wanted to get seriously weird, you could swap steering columns as well. The 1968 had its ignition switch on the dashboard; the 1969 had it in the steering column. Depending on how you combined steering column with year of car, it was possible to wind up with a LeMans that had two ignition switches... or none at all.)