You can try it and see, not a big investment. Just get under the car and see what the spring perches look like just to confirm.
Secure the car from rolling. Simply unbolt the shock at the bottom and jack up the rear of the car high enough until the springs can be pulled out. You may want to install new rubber insulators at the tops of the springs while out. I believe the spring pigtail at the top of the springs, along with new isolator, may have to be "clocked" to fit into the pocket. Look up inside and you may see where the spring pocket has an indent (much like the front springs lower control arm pocket) and the end of the pigtail will set in it. Then install the spacer, put the spring on top, drop the weight of the car on the springs to compress, and bolt the shocks back up. Done.
I think that covers it. Your mechanic, if he does it, should make fast work of it with all the correct pieces. If you don't like it, then go new springs, but I would get a custom spring company that will make a spring for your application. A new stock height spring might get you right back to where you are now.