If you are concerned about the 4" wide rims with 6.50 x 16 tires on a '40 Ford, switch the rims to '56-66 Ford pickup wheels. The pickup wheels will take a similar style of hub cap as the '40's, however, they will be 5" wide. Most of the later model pickup wheels will be 15", however, a lot of the pickups had the 16" option for 6.50 x 16" 6 ply tires.