Now that the work is done and I've got the car on the road, I can report the following:

I set the lower bars parallel to the ground, 25" in length (that's what I had on hand). I set the upper bars at 45 degrees, slightly above level. I made all the mounting flanges out of 3/16" wall rectangular tubing, so they're strong enough. I used T-style rod ends, not Heims.

The rear end is very flexible up and down, but very stiff side-to-side; exactly what I was looking for.

I also agonized over locating the differential housing side-to-side. The housing is not centered, but the pinion is, so I just went with that, also set the pinion angle parallel to the output shaft.

Finally, I took pains to get everything to less than 1/16" of perfect, which made final adjustments much easier and the rods just drop into place.

Oh yes, the car rides and handles just great. Thanks again to this and the other forums for advice and links to more advice.