"Hot rodders" hits the nail on the head. We 21st century dudes have a hard time imagining what life was like in the 30s and 40s -- but in short, it was almost like the fates conspired to set up the perfect environment for field modifications.
In the early 20th century, mechinazation and engineering felt every bit as cutting edge as computer technology does for us now. Rapid advances, new inventions, reshaping the world, the whole bit. This is the era that Popular Mechanics magazine was created -- and try to imagine that it sounded like PC MAgazine to them, not like a journal for grease monkeys.
Then toss in the depression. Lots of people living on farms, lots working week to week hand to mouth. Reusing, making do, penny pinching, do it yourself stuff weren't lifestyle choices, they were the only way to survive.
So an entire generation grew up tinkering, modifying, kitbashing. They'd take an old gas engine and rig it to do something else, like run a pully system to get the hay into the loft faster. They'd bang stuff together to make parts into a tool they couldn't afford to buy. And if they did have a little money, they'd be ABLE to mess with the settings on their cars, because they didnt have to worry about fuel injection and computer chips under the hood.
Lastly, consider this -- as a nation of immigrants, Americans in general had a higher chance of having adventurous, aggressive, risk taking genetic tendencies. Modern science has found that a good portion -- as much as 50% -- of some personality traits can be traced to genes, not to character strength or upbringing. Folks who were optimistic enough, bold enough, risk taking enough to leave everything and start over in the New World tended to have more adventurous personalities than those who stayed home. THAT has a lot to do with the American "can do" attitude, as much as the nature of the new world itself.
Mix those together -- cutting edge engineering environment, "can do" personalities, a decade of tinkering and mechanical work -- and its a "perfect storm" for field mods, from the gunships to the Normandy hedge cutting Shermans.
'Course, nowadays we're perfectly set up for kids whose thumb dexterity needs to be high...
