I believe the Mt Vernon decals you are looking at are in O scale for 1 car.
There are also decals for "steel cabin cars" in N scale (N-PNS) that will cover 6 cars.
These are for pre circle keystone, but you could possibly doctor them a little to suit your time frame?
My memory is probably not serving me well, but I seem to remember that only N8 cabin cars ever received circle keystones?
John has indicated that he would run any of his decal sheets in N scale if someone put together an order for 10-20 sets.
hope this helps.....