Oh lets count the ways
1. If you try to install the torsion bar with the UCA and spindle attached, the LCA will not hang down far enough to easily get the torsion bar into the LCA socket. If you leave those attached, you have to remove the upper bump stop, otherwise the LCA won't hang low enough, even with all stock parts. It's literally in the FSM.
2. If you're not using a stock torsion bar, the hex offset on the bar may be different than stock. Which again means that with the UCA and spindle attached, you might not be able to get the LCA socket into the correct position.
3. It's a zillion times easier to manipulate the position of the LCA without the rest of the suspension already attached
4. Depending on how long your UCA bump stops have been installed, and their quality, you may spin the stud in the bump stop, and therefore ruin the UCA bump stop trying to remove it.
With the stock K and LCA, the LCA hits the K frame before it hangs down all that far. With tubular UCA's, it's pretty much required that you clearance the UCA mounts so full travel is necessary. This is required, not a trick to get the suspension to hang lower. You shouldn't have to clearance anything to get a tubular LCA to have more range of motion than it will have when everything else is installed.
I've installed stock torsion bars and aftermarket torsion bars, with stock UCA's and LCA's and aftermarket UCA's and LCA's. By far the easiest way to install (and remove!) torsion bars is with the LCA unhooked from the spindle and UCA. By far. So that's why I do it that way.
