I did it, but I also rebuit everything in the rear at the same time, including trailing arms, rear end, brakes, and parking brakes. This was a lot easier then with everything out. But, I think it wouldn't be a big job. Just put the car up on jack stands and "GET UR DONE".
There are pro's and con's to them. I only use steel on my cars and the aftermarket steel springs are not arched correctly.
1- cost much less
2-look somewhat stock
3- last longer
1-Not arched correct- car sits 2" high with the stock 6.25 bolts
2- require 8" bolts to bring the bbody down.
3- 40 lbs heavier
4 rust if not painted
5- have to be radiused if used with a HD cover or the 4 mounting bolts hit and roll the threads.
1- 8 lbs
2- handling is better- I think this is personal opinion
1- cost 2-3 times that of steel
2-Break - main reason I won't use them
3-require new shocks valved for them or you'll be bouncing all over the road
4 still require 8" bolts to adjust the body
I have the VB & P dual mount spring. Wow, what a difference! Handling is noticably better - the glass spring seems to react much quicker than the steel. I have the QA1 single adjustable shocks to help fine-tune damping for my locally rough streets. Less weight is always a bonus in performance arenas. Changing the spring only took about 5 hours with basic tools (electric drill being the most advanced). As for steel, they can break, too. That's the reason I needed a new spring. I don't think breaking glass springs is a major problem or GM wouldn't haave used them exclusively on our favorite toy for the past 23 years. Your call - I can say the fiberglass front spring is next on the upgrade list on my '74!
I have had a fiberglass monospring in my 72 all summer. Prior to that I had an incorrectly arched steel spring that made the car sit high in the back (even with the 8 inch bolts). The fiberglass spring, in my opinion, rides much better than the incorrect spring I had prior. As space cowboy said above, it is a much smoother ride in my car.