I've pretty much answered this question in the Bloopers thread. The narrow push bar was an off-the-shelf part for pickup trucks and was only welded to the bumper. The wider push bar was custom-made and welded to the chassis making it much stronger.
If you click the link you'll see where one of the narrow push bars fell off at the start of 'My Son, Bo Hogg'.
Why did The Dukes of Hazzard General Lee 1969 and 1968 Dodge Chargers change from the small push bar that was used early on to the wider push bar later on?