Bruce, What did you end up using for the side sills. I have a whole bunch of these to do as well. Ben
On the ones I have done, I used the Rivarossi cars so I just left the plastic side sill in place on the shortened core. On the newer sides from M&R like the fish belly baggage and all the chair cars, there are etched side sills provided. For these sides without, I suppose you could use some of the extra metal that is on the edges of the etching sheet to fashion some side sills.
I've decided I'm going with styrene strip...it can also be very securely cemented to the styrene body shell as well as affixed to the brass side...the only reason I haven't finished these three is that I've acquired 5 more undec MT cars and an example of the new horse express, so I'm just going to go ahead and build them all assembly line fashion. Bruce