Spindels?

B.R.E.Demon said
I'm planning to shorten the arm a bit more, bringing the total length to 8 inches. With all the boxing in, I don’t foresee any potential for failure. I’ve also explored the idea of creating my own custom lower ball joint mount from scratch, designed to accept a properly tapered, threaded ball joint. One thing I've learned about Mopar is that while the design is great, many choices were made to reduce original production costs and make parts applicable across multiple platforms. For this reason, I prefer not to introduce heat to the ball joint casting by welding on it, as it seems to be cheaply made. The final design will be fully boxed in, similar to the one in the picture, with small sections welded to connect the webbing, making the arm a single piece.

Have you looked at LCA's from the 73/74 B bodies? They mount the lower ball joint in the LCA directly and have a steering arm with no ball joint. The steering arm doesn't help you but the LCA might. Either way you'll still need something to hold the ball joint to the knuckle
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But again, I'm not talking about welding on the stock steering arm. I'm suggesting that you should bolt your fabricated steering arm to the knuckles, because the way you're going to weld those arms to the knuckle will not be as strong. The rest of the construction isn't what I'm talking about either, the weakest point of this entire arm is right here at the red line, where you have designed in a hinge point with a large lever.

B.R.E.Demon said
The advantage of using heims in a fabricated design is that if you run a smooth OD bolt into a taper, you only have a small contact patch, which puts all the clamping and transitional force on the bolt—a potential point of failure. vs the continual contact patch in the fabricated arm.

No, I get that. I'm saying you don't need to weld the fabricated steering arm to the spindle, because nothing changes for the rod end. You're still fabricating the steering arm, you just bolt it to the spindle because that will be a stronger arrangement

B.R.E.Demon said
As for the 9-inch spindle, I switched from the '73 and later disc brakes years ago in favor of these spindles. They're lighter and can be further milled to reduce weight. Aftermarket brake kits for drum spindles are also lighter. Bearings aren’t a concern, as we use the same series of ceramic bearings found in top fuel and funny cars, which are even smaller than ours.

Hey if strength and bearing size aren't a concern for you then have at it. Personally I can't see racing one of these cars so competitively that a few ounces on the knuckles would be worth the loss of strength, but that's just my opinion. A knuckle failure at speed would be catastrophic