I'm using a 67 12 bolt in a 70 Chevelle. What spacer will work the best for a 1/2"? I was thinking about the shim type that fit over the studs and installing longer studs to make up the difference.

IMO, 1/2 inch is too much for a spacer...

Is this the same car in your signature (one that you race?). If so, I would definitely rethink the use of spacers... With 1/2" of difference, I think you need to look into different wheels or rearend... With 1/2" of unsupported length, you will be introducing alot of bending stresses into those studs, not only from regular driving, but from dragstrip launches. They are designed to handle mainly axial forces, and while they may support the weight of thecar, the cyclical flexing will likely cause them to fail...

Maybe if you found a spacer that would index off the axle stub and into the wheel, or at least go to larger studs (and maybe better material like ARP studs, etc)...

This rear is going in a 307 malibu I just got. I was thinking that 1/2" may be to much. I have a set of 14" and 15" Rallys that I was thinking about running and wanted the back tires to fit right.