So you will be using the new bushings that have a 12 mm ID and the OEM-correct 12 mm bolts.
but using those 12 mm bolts and 1/2 inch bracket holes will, as you know, leave the potential for movement. I do not believe the forces at work in the suspension of a 3000 pound car can be overcome by tightening the 12 mm mounting bolts.
I believe you could use the adapter bushings I posted earlier without actually welding them in place. For that action I believe tightening the bolts would certainly be sufficient, and safer than allowing your suspension to have 16 points of potential movement
I would certainly advise welding them in place at the earliest convenient moment.
Larry