I'll agree that the brackets are an afterthought in the design. Obviously production costs outweighed designing a bracket that would seperate but also go back together to replace the bushings.
There is a procedure for replacing the bushings. To seperate the two halves of the bracket you have to drill out the flare on the back side. Here is a picture to illustrate:
http://www.flickr.com/photos/bigtimes_fiat/2473059952/
The two halves are swagged together with a flare. I think a 1/2" drill bit will work. The problem is that you almost have to drill through both pieces and usually end up with a 1/2" hole for a 1/4" stud.
When you put the whole thing back together you can assemble it on the car. You may want to clean up the threads on the studs first as they are usually rusty and could strip the nut. First put the new bushings on the bar and slide them to the general location where they should be. Secure the two outer bushings at the "A" arms first. Put the smaller rear bracket piece on and then line up the bushing. Put the outer bracket on and don't forget some washers.
Here is the hard part. If you can push up on the bar and get the nuts started then you should be pretty good. If you try to tighten the nuts to bring the whole assembily together then the pressure exerted by the bar and the rubber bushing will strip the threads off the stud or the nut.
You need to somehow push up on the bar or brackets and then tighten the nuts. I had to place a jack under the bracket to push it up so I could tighten the nuts. I had the whole weight of the front of the car on the jack under the bracket and it still did not completely come together.
Some people will assemble the parts on the bar, then use a clamp or vice to squeeze the brackets together. After that they tack weld the two brackets together where they touch. I didn't do it this way because I could not move the origonal bracket that was on the bar. If you dont assemble it on the right spot on the bar then it is very difficult to move once its clamped together. I also felt that the heat of the welding would melt some of the rubber.
If you clean up and paint the bar prior to installing the bushings and use some WD-40 the new rubber bushings will slide right on. If the bar is rusty, expect to fight the bushing all the way to where it is supposed to go.
Matt