I could tell you that a stock 1940 Pickup is 14 1/2" with stock 16" wheels and about 60 years of service. But, try this link:
http://www.ford-trucks.com/ and ask the boys in the 47 to 51 section. I don't know what purpose you need to know that for, but you could figure it out your self by placing a jack under the front axle and jacking it up until the door sills were level (use a bubble level), and then measure down from the front horn to ground. Tire pressure, spring sag, etc all have to do with this measurement, nothing's perfect!