Jim, I don't think I'd worry too much about where the wheel ends up in the extreme travel positions unless clearance is a problem. When I made my control arms, I used a straight "mandrel" to position the wheel to be perfectly in line with the axle centerline of the trans at ride height, so that the IRS joint isn't "working" any more than it needs to. The right way? I don't know for sure.
As long as you start out square, it'll stay that way unless your trailing arm bends. And if you leave a few threads on the rod ends for adjustment, you'll be good.
Billyisgr8 makes a good point. You will want about 1 degree positive camber built into the arm to allow for flex, maybe just a smigin more so that the wheels are at zero camber at ride height.
You'll still end up with a couple degrees camber change throughout the travel because the arc is not at a perfect right angle to the torsion housing. So if you run it lower that where you do the initial set-up, the camber numbers will vary slightly.
Build your diagonal arm straight like you suggest, then move the completed arm throughout it's travel and watch what happens at the rod end. Now on my photo, draw a straight line from one rod end pivot center to the other. That is the reason for the "stub" that the diagonal is attached to. Even then, it's still a compromise when the stock torsion housing is used to locate the pivot points. But for three inches of suspension travel, it's not too bad.
It's a really good thing that you are asking the questions, because there's more than one way to "skin a cat." Maybe you can come up with something no one has thought of before
I didn't make it the way I did by accident. What I ended up with is pretty far removed from my initial design I had on paper. Keep going Jim. I'm very interested in what you come up with. If it's better than mine, I'll steal your design fair and square and re-do my own arms!
When you're building your own stuff, you can pretty much use whatever shock fits in the space.
I ended up with my lower shock mount much higher than stock so that I could bring it as far outboard as I could go and keep it out of the way of the CV joint. I just couldn't make it work with the shock in the semi stock location without making compromises on the trailing arm design. I thought it would become too flexy or too heavy if I worked around the shock. I was able to make 'em stiffer AND lighter by working the shock mount around the arm instead. Also if the lower shock mount is too far inboard, you'd need stiffer springs to deal with the leverage factor. Which brings in another factor. If the spring needs to be much more that 500 lbs, springs are only easily available in shorter lengths, like 7".
I got mine through Mark Williams. I also understand there are others out there who make a custom length IRS axle. Rancho could probably provide axles through one of their suppliers. I have heard that Sway Away makes 'em and a place called KarTek in so cal somewhere. The beauty is that you are not limited to either 3" per side or 5" per side.
Mine were fairly pricey at $800. But it's something I'll never have to worry about.
Jim, I think you should put a wheel & Tire on before welding this up. Looks like you already have a custom upper mount. From the pictures, the shock looks pretty low. Something to consider would be raising the upper mount so the lower mount doesn't drop down so low. I ran into the same problem. I ended up raising my shock mounts, as I also needed clearance for coil springs. You probably won't need to go that high. Maybe just as high as the body will allow.