Note that OEM 12mm bolts were the metric equivalent of Gr-5. Nowadays, 12mm bolts can be had in Gr-8 if you're concerned about strength.
It is critical that you weld the step washers to the suspension mounting tabs as you’ll be relying on the washers for strength. The inserts that reduce the ID of the holes will only be 0.013”, which is paper thin. Any movement and they’re gone. Have you considered just welding up the holes and re-drilling them to 12mm? Welding on those tabs won’t be easy as some of them aren’t too accessible. Even cleaning them up for welding won’t be easy. I still think the easiest way to solve your problem is to enlarge the holes in the new bushings to 1/2”.
Alrighty. Update time.
I took the bushing sleeves (poly so they pop out) to my local machine shop. He is going to ream out the inside diameter from 12mm to 1/2”. I also ordered some aircraft bolts with grips long enough to pass from hole to hole, which I think provides the appropriate rigidity.