The squeaky wheel: what does it get?
 

Basically, these are new brakes (front discs, from early 70s A-bodies) on my '66, but the right front one squeals embarrassingly, unless I apply the brakes. Any idea what's going on?

Could just be a glaze on the pads or rotors.

A squeak that goes away when you apply the brakes is often caused by vibration. The pads are always in light contact with the rotor so when driving it causes the pad to vibrate. Get some anti-squeal adhesive at the parts store. Clean the back of the pads and caliper and apply the adhesive.

That's the cheapest solution, if it works. Sometimes it's tough to get rid of the squeal and changing to a different type of brake pad is required.

The noise may be annoying but the brakes are still safe.