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There is a thread about replaceing the rear axle bushings titled "Removal of Rear axle bushings" that is very good. It was started on 08-09-2016 at 10:13 PM by ULOS2A6 check it out it may help..
About what was the cost of the rear disc brake conversion, and where did you locate the kit?
Junk yards, internet are a great way to track the parts down. The HHR/Cobalt have been out of production now for a while so there should be some around in junk/salvage yards. I used a website called car-parts, its a junk yard website for parts, great way to track it down.
X2 with car-parts. ebay is also useful. I found my axle beam in St. Louis. Bushings and hubs - MOOG, brake discs, pads, rotors - PowerStop, brake lines, parking brake cables, hardware - Raybestos. Total cost of $850.00 plus time to install. Oh, and $200 for bushing tool J44570, which I now have and will let others use for the cost of shipping.