I was looking for something else and found this on the GM Heritage site.
1949_Chevrolet_Truck_Engineering
it goes into detail on a couple things relevant to this thread, including the switch to shackle mounts in the rear, but mostly the change of the lower core support mount from center mount to 8" apart and the addition of the x brace in the rad core to stiffen the front clip shake on rough roads. it doesnt say it specifically, but logically that has to be what the underhood braces were for.
interestingly enough, it talks about the flipped door regulator rods too, which was a discussion on here a while ago.