Yes, you DO want to loosen (NOT remove) the bolts with weight on the car.
The actual load on the bolts is not great in that case and it does open up
the space so you can install shims.
If you are changing the caster equally on both sides you add shims under the
crossmember but if only on one side you do it under the upper A-frame mount.
You almost have to do it this way because if you take it apart each time it
may take several iterations to get it right and it gets pretty tedious.
Also if you don't have a balljoint press you might ruin the ball joint boot
taking it loose from the spindle forging.
(I still have some ball joint presses that work on both balljoints on 140/164
and the lower one on 120/1800. Remove them without damage. E-mail me offline
if interested.)
--
George Downs, Bartlesville, Oklahoma, Central US