Just because the spanner is far down doesnt mean there is alot of preload. It could just be where it is resting since it is a longer shock.
I just built one of these truck, it is currently sitting in my shop and I can guarantee that a normal 8" will not work with an extended rod end. OP, take the shock off, pull the spring off and cycle the suspension, If the shock bottoms out way before the uniball binds out, you will have your answer right there.
Before you go hacking apart your coil bucket, try putting the top mount on upside down to see if it will even clear.