'69 Barracuda disc brake swap using m-body brakes?

Yes, big ball joint 73-76 upper control arms or aftermarket tubular if you want to add some camber. As far as using the knuckles from the fjm, there are lots of guys on here doing it with no problems. I have a 70 ebody setup for sale and the geometry of the spindle is the same as the 73-76 a body spindle if your interested.