I just went through this myself, and was going to post some tips.
Basically, you need to expand the chassis opening a bit. One person recommend using a 3/8 bolt with a few nuts and washers. With the wishbone removed, put the bolt partly through the chassis opening through the bushing bolt hole, put the two nuts on (in the space where the bushing goes, and thread them up the bolt so that the bolt goes through the second hole. Then use the nuts to spread the opening. Not too much (you are just trying to create a little outward bend). Then remove, and the opening will be slightly expanded. The nice thing about this approach is it puts the force of expansion on the reinforcing black steel pads.
The other way I used (sometimes in combination with the above) as to get a 5/16 inch diameter short bolt (e.g., 1 inch or a bit more in length) with a long 5/16 inch coupling nut (normally used for coupling two thread rods) I screwed the bolt all the way into the nut, and with the wishbone in place, put the combo in the opening somewhere near the middle as close as I could get. Then, unscrew the nut/bolt combo to put expansion force on the aluminum walls. You will then be able to slip the bushing into place.
I have a attached a few pictures. The second one, using the expander made of a bolt and a coupler nut, shows a 3/8 inch diameter bolt version. However, that nut was really a bit too long, and I went down to the 5/16 inch diameter nut which was shorter.
I intend to write up some more tips in another thread, but that should get you past your current problem.