Will the RS-3 body from RTR Set 6-82984 fit on a newer RS-3 LionChief plus? And will its lighting connectors plug-in?
The RS-3 is a good bet for running with smaller passenger cars, because of its lower hood design. The engine is 1 1/2 inches longer than the Baby Madison cars, but still looks good with the cars. Other than the Lionel small Alcos, the RS-3 is about the best you can do, and has a perfectly satisfactory appearance at a pretty good price. Just my opinion, of course.
I have swapped shells on an older conventional RS-3 with a LionChief Plus RS-3. Don't recall specifics, but it really wasn't that hard to do. For LionChief conversions, the RS-3 seems the easiest to do. I have a second one I'm about to do a conversion on as well.
As an aside, I've looked at doing a LionChief plus conversion with GP7s/GP9s, including examining using a LionChief plus Recifier chassis, but those appeared far more difficult to do, so I haven't attempted that as of this point, and probably won't.