Use as much as you can from a donor, within reason.
It helps keep costs down, and you remember the application easier!

As far a ride height, even a full gas tank can throw it off. That's one of those things yo uhave to wait on setting until it's a lot closer to done. You can block up the suspension on a coil spring to where you want, then order springs using the final weight, but with leafs it's pretty hard to approximate exact height.