8 1/4 rear end swap

Welcome to FABO, you need a 8 1/4'' rear end out of a '73 to '76 a body.
Take the rear end with the brakes, u bolts, and shock plates.
The drive shaft is longer on the small rear end you have, so it needs to be shortened or a drive shaft from the parts car might be the right one, as long as the transmission and the body style is the same.
As for the disc brakes, the car you remove the rear end out of would have all you need if still there.
I would take the upper control arms, spindles, calipers, caliper mounts, rotors, master cylinder, and proportioning valve. Inspect the parts and rebuild whatever is worn out of spec.