American soldiers weren't fat, they were in their late teens and twenties, and fit; and looking back on myself at that age, I wasn't fat either! Middle aged spread hasn't been to kind to me in recent years though!
I saw a picture of a jeep that had the drivers seat supports cut, and extended rearward by about 6 inches. This also involved cutting into the rear wheel arch to accomodate the seats new backrest position.
There is no built in adjustment.
I can still fit behind my jeep wheel, but because of my long legs, I have to get my right knee under the steering column before sitting on the seat, or it won't fit past!
Steve