sooner; According to thr 1939 Engineering Features -Conventional Book--Quote--Partly because of the 1-1/2" increase in wheelbase and partly on account of changes in the fuel tanks and their connections, the gasoline feed pipes on all 1939 trucks are longer than they were in 1938. On the Light Delivery and 3/4 Ton Truck chassis without cabs, the gasoline feed pipe connection is relocated so that the same feed pipes can be used on all these models.
It looks to me as though the '38 won't work.
Hope this is what you were looking for.
Stan Norrish