Actually left-foot braking is one of the advantages to an auto, and one reason I always go with an auto over a stick. By using the brake much as one would use the clutch you accomplish the same thing, but you get more power to the non-spinning wheels and it tends to help keep the spinning wheels from spinning. Also, if you need some extra torque to climb something, you can build up torque by power braking. Also, there is no lag time between moving your foot from one pedal to the other....
anyway... the reason I wanted to make the pedal larger is actually just to make it easier on my knee. The pedals are designed to be used with only the right foot, keeping the left foot for the clutch. So the brake pedal is in between those locations... which means it is not directly in front of either leg comfortably. I sit very close to the steering wheel (an old habit from my mustang - it was basically set up like a race car) so the angle for my left leg on the brake pedal is enough to fatigue my knee after a long day on the trail.
I'm not a very good welder, so maybe that would be a bad idea if it would weaken the metal. I'd rather not move the pedal because I often loan my Jeep out for friends that don't have cars, so I'd rather not make it difficult for them to drive. (and I imagine that is more involved than it's worth.) Would a wider pedal put too much leaverage on the pivot? I wonder if I could get a clutch pedal and install it for support and attach it to the brake pedal? (I don't know if that's clear... it would be like a two-hinged pedal, but only attached to the brakes on one side.) I assume even the auto XJ's have the attachment points for the clutch pedal?