Fastback roof a-bodies and the 67-69 barracudas have a shorter wheelbase (108) than the square body Darts and Scamps (111). Also the dimension from the center of the pinion yoke back to the center of the axles is different between each type of axle they made.
Any issues are likely going to be the driveshaft length and the pinion yoke size possibly. I don't know if the u-joint sizes are different between 7.25 and the 8.25, but it would not surprise me if they were not the same.

I'd put in in the car and then see what needs to be done with the driveshaft + u-joint size and go from there.