I'd start with a small drill bit then move up. The small bit should get the hole started without flexing it.
Have you welded the gussets in the control arms yet? If not, I'd do 11 or 12 gauge steel. No less. Thin steel isn't worth fooling with.
Have you welded the gussets in the control arms yet? If not, I'd do 11 or 12 gauge steel. No less. Thin steel isn't worth fooling with.