I am not sure that 28/29 and 30/31 wood strips are the same. 3" seems quite wide from what I have see. I thought they were not near as wide, but that is just what I remember. What year roadster do you need the dimension for? I got the information from Ron Rude some time ago for a 28/29 roadster. I have no idea whether the dimensions are correct, but Ron usually knows what he is writes about.
Rusty Nelson
Dimensions for mounting wood strip for the side upholstery panels (from Fordbarn - Ron Rude).
Made from hardwood like oak or ash.
29- 3/4 " long
1-1/4 " wide
5/16 " thick