6464s like Timken, Rutland, WP Feather, Alaska, Pennsy, B&O Sentinel (the B&O Timesaver blue and orange car overlapped), Rio Grande, NH - you sure those wouldn't look good in S?
Some Flyer boxcars O gauge guys might like as O gauge cars would be the red GN, L&N, GAEX, G. Fox - those cars would look great in different scales. I'd suggest painting them with appropriate numbers from their respective manufacturers (6464 on the Flyer cars, 900/5 digit numbers for the Gilbert cars).