I have the J-bars installed on my Duster, and the US Cartool inner fender braces on my Dart because I wanted the chassis work to be less obvious on that car (matching numbers GT). I have the magnumforce bars for my Challenger but haven't installed them yet as I still have chassis repair to do on that car.
I do think they're important. The front half of these cars, especially from the cowl forward, are pretty floppy. Part of that is because they can be, the suspension loads are carried by the K frame and crossmembers so the vertical plane didn't have to be as stiff. I noticed on both my Challenger and my Duster that you can actually see the hood and front end shifting around on the road. "Cowl shake" I've heard people call it. Anyway, I think on anything with upgraded suspension and tires that section of the car needs some reinforcement. The J-bars are probably overkill, the US Cartool pieces are less intrusive and still triangulate the upper shock mounts, cowl and frame rails. I actually think that and the torque boxes are probably the number 1 and 2 chassis improvements, the subframe connectors are important but I think the overall stiffness of the chassis is improved more by the torque boxes and forward braces.
The bars on my Duster are 1.75" x.083 or .120", I don't remember. The magnumforce bars for my Challenger are 1.625"x.120". Both have provisions to connect them to the cowl, the upper shock mounts, and the frame rail. On my cars I also have lower radiator support reinforcements, so like on my Duster the J-bars are tied into the frame rails above a tubular lower radiator support brace. So the bars go a little farther forward than the bars on the "Red Brick". They're also higher above the inner fenders, which makes them a little more in the way. The magnumforce bars are fit tighter to the inner fender and that would be an improvement over the Dillinger ones on my Duster. At least cosmetically and for working around them.
I don't think it makes a huge difference if they're not part of a cage. They're being welded to sheet metal at the cowl, and even with a large landing plate I think the strength of the bar is much higher than the connections to the cowl and frame rail. So I wouldn't spend a ton of time thinking about what size. The 1.625" x .083" should be plenty strong enough connected to 3 points with large landing plates. I don't think I'd go too much larger in diameter just because the larger the diameter the harder they'll be to work around and fit tightly to the car.
