Again i'm not familiar with the 40's type cars but i know on muscle cars, lets say a chevelle, you can get away with running about a 11-12" tire by simply cutting out the stock wheel well and replacing it with "mini tubs". then just play around with the wheel backspacing and about a 12" tire can be put under it without toching the frame.

But like i said you will need to look at your car and see how much room you have avaliable to put a mini tub in there and then figure out the back spacing and see what you can fit.

Plus allow a bit of room for tire groth at speed.

BG