I like to use 5/8" threaded rod as a compressor. Put it where the shock would go. If you need any more info or pics I would be happy to supply the info. It works good and it's a little safer than the spring compressors.
We used the method Jim describes above to remove ours and it worked... but ended up renting the smaller inner compressor from Autozone to put the new ones in. Make sure you put a safety chain on... those things can do some damage.
my cousin once was working on a 63 impala ,he compressed the springs in his high school auto shop chained them up and took them home on the bus.this was 25yrs ago.luckily all went well.
I use the 5/8 threaded rod they have a Home Depot. Just the regular stuff. The tensil strength of the rod is twice what it will take during compression. Be careful not to damage the threads. To compress I use a piece of 1/4 to 1/2 inch flat stock on the bottom and double washers on the top. I also double nut the top. I disconnect the lower control arm so it doesn't interfere in any way. I detach it at the ball joint while there is some spring pressure and I detach it at the bushing after I have let some spring pressure off.
We put the plate in the lower section of the spring when we removed ours using this method. Also... grease your threads before you start. Makes it a lot easier. Dont forget to use a safety chain!
That is the way to go. I've remarked on this method (with photos) several times before. General purpose spring compressors are general purpose and can be dangerous. If a spring compressor slips and cuts loose, you have a lethal projectile launched in your garage. Springs under compression are dangerous to mess with. I think this is the safest way to work on them on the Firebird. A threaded rod through the center of the spring won't cut loose like a dislodged spring compressor can.