If you want to use a regular "H" frame press, you need to block up the A-arm between the bushings. Otherwise the arm just collapses and puts a bind on the bushing.
Here is a pic from a ford manual showing the blocking setup. Chevy can use the same setup.
Here is a set of blocks for chevy a arms I made out of muffler tubing and some scrap steel., The U shaped blocks go in the channel section of the lowers to keep the end from squeezing the bushing.
The cup shaped things with the plate welded on is what you push the bushing into.
I've made those spreaders out of wood too..
You'll need the spreaders to help install the new bushings as well.