There is a few way's to accomplish this.
One way that is easiest to manage without risking hurting yourself is this. Place the strut assembly back into the truck. Get the nut's started on the top plate stud's an the bolt through the lower strut eyelet. You don't need them tightened, just finger tighten the nut's to the stud's an bolt. You can now use the open end of a box wrench to hold the top of the strut on the flat spots. I believe a 6mm wrench is what held my 5100's still. Hit it with penetrating oil, use the 6mm wrench or vice grip's, while getting the nut started loose with a wrench. Ratcheting wrench would be ideal.
From the point it starts to loosen, you can either place a jack under the LCA, lift the LCA until the truck almost come's off the jackstand under the frame rail, so the suspension is "loaded". While it's loaded, the pressure come's off the spindle an UCA. You can loosen an remove the two lower balljoint bolt's. Once those are out, you can fully remove the top nut from the strut. Then, SLOWLY lower the jack under the LCA, this will let the pressure off the coil spring without using your compressor an the LCA will separate from the spindle as the UCA pulls it upward. The LCA will swing down vertical to the ground an you can safely drop the strut down an the shaft will swing towards you to remove the spring. Swap them out, then slowly jack the LCA into place paying close attention to the strut shaft an the hole in the top plate. When you line that up, jack the LCA back up into "loaded" position with vehicle weight on the jack. You can now tighten the nut down onto the strut shaft, tighten the top plate nut's, tighten the lower shock bolt, the LCA will be back up against the spindle/hub, line up the balljoint retainer plate an thread those bolt's back in.
The LCA cam bolt's get 100 ft lb's (you'll need an alignment which I'm sure you knew), the lower shock bolt get's 100, the lower balljoint bolt's get 116 ft lb's. This is safer then any spring compressor in my opinion, plus you don't have to fight loosening stubborn hardware on a loose object that can roll around. Hope that makes sense from my head through the keyboard. I can throw a front end together in minutes on these truck's now. I may have skipped a small step, but you seem like someone who will pick this up quickly once you start seeing it come apart.