If you want to go 4x4 in the future I would already start with the full 4x4 setup, as mixing 2wd and 4x4 parts is not going to work. The arb is a different length and I think that should also be combined with the correct crossmember and correct hubs. There is more to worry about than just the roll center, the caster and position of the wheel in the arch is not going to be correct when you combine 2wd and 4x4 parts. So you should already use the 4x4 crossmember, steering rack and hubs from the start. Besides that I would use the Cosworth 4x4 hubs. They have a better roll center than the XR ones and don't push the wheel out of the arch like the XR ones do. I would also use the 28 mm Cosworth arb instead of the 26 mm XR one, although later XR's might have had the 28 mm one as well.
In your situation, where you don't want to alter anything with the TCAs, I would just buy a good set of TCAs and not be messing around with the ball joint.

But why do you want to go 4x4 anyway? In my opinion the V6 doesn't have that much power that it benefits from 4x4.

To come back on the rear driveshafts. I've just had mine apart and the splines are exactly the same length on both ends of the driveshaft. It also doesn't matter if they aren't as there is lots of movement available in the CV joints to compensate for the small difference you have on the splines.