Most NG equipment that I have found throughout my modeling career has a 7 1/4" board, ship lapped at around 1/2 inch. This means that all but the end boards will be 7 1/4 wide on top and bottom, with about a 1/2 inch offset in the middle. The end boards will be ship lapped on only one end. Total board width would be 7 3/4", possibly planed down from a 2x8. Wood is about 1 3/4" thick. This is true for the 30' cars.
As for the fishbelly flats, I am not sure, but I would believe they aren't any narrower. Somewhere I seem to remember seeing a picture from later on in Rio Grande years where it looked as though there was no consistency, and that most of the wood was completely random sizes just thrown on to replace what had been broken or rotted over the years?
Casey