The top nuts on mine, at least, were not really rusted in place. Regardless, getting a wrench in there to tighten the new ones is about the same battle.
I found a flex-head ratcheting box wrench to be the handiest tool. As required per Murphey's Law, the replacement shocks used a different size nut than the old ones, so I had to buy two such wrenches. Some years and some chassis have rubber plugs in the foot space from the inside that can give somewhat easier access than trying to reach around from the bottom...but not all.
The improvement in handling from the old, no-longer-absorbing shock absorbers to the new Koni FSDs was not subtle. I'm a happier driver now. I also don't plan on doing the job again merely for the fun of it.