Longer answer, discounting composite and side-dump cars and mill gondolas:
N&W cut down 800 class GJa gondolas to 3' sides in 1934-5, #s 98000-98799 class GJc.
N&W cut down 500 class GU gondolas to 3' sides in 1938, #s 90000-90499 class G-2.
Virginian cut down 75 class G-2 gondolas to 3'6" sides in 1941, #s 3850-3924 class G-2A.
N&W class G-6 and G-9 were longer than 40'.