65 Barracuda k-member swap? & rearend

You are correct on the K member. The early A bodies ('63-'66) used the same K member for slants and V8s. Any A body 8 3/4" rear end will bolt into your '65. 1965 was the last year for the ball and trunnion drive shaft though, so a '66-'72 rear end will require the slip yoke style drive shaft. This will require a '66 up output shaft and tailshaft housing on your transmission, a '66 up transmission, or a '65 or older center chunk for the rear end.