Well just as the topic reads. Will a set of disk brakes from a 72 Chevelle fit a 67 Firebird with manual drums?

67 firebird uses the same brakes/suspension as a camaro.IIRC this can work but requires modifications to work.67 F-body is a rear steer car.I think the steering arms have to be installed backwards to make the A-body spindles work,but there may be other issues.Maybe one of the Camaro guys can chime in.

You can reuse the drum brake steering arm from the Firebird on the Chevelle disk spindle.

Like DG said, use the steering arm that is on the car now, you can't just flip the Chevelle arm around for the Firebird, they have different drops.
The calipers are slightly different where the brake hose attaches, either will work and either will fit the caliper bracket. So if you're getting rebuilt calipers to go on, just order the Firebird ones and Firebird disc brake hoses.
You will want to use a Firebird master cylinder because there could be hood clearance issues.