I just bought a 71 Dart and it has a 7.25 small lug pattern rear end in it.
I found a 8.25 for sale but it came out of a 74 Duster.
Will this fit in the Dart? I'm new to A bodies and I don't know if there is any difference in the rear end dimensions.
The Dart has a 318 and 904 and is going to be a cruiser. I'm just looking for something a little stronger than the 7.25 and the 4.5 inch lug pattern.
Thanks for your help.

Pretty sure it would bolt right in.
U-joint may differ, but I believe the A-body floor/frame architecture was pretty much the same '67-'76 as far as frame rails, suspension mounting, & such.

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.

It will bolt into the springs. Wheel bolt pattern will be different. Driveshaft length will be different probably too long, IIRC, odds are good the U joints are the same, 7260 type.

I can't remember if the shock mounting plates and U bolts will work from the 71, just use the 74 ones. Shocks will work.

The Archives have that info.

http://www.moparts.org/Tech/Archive/axle/5.html

You should be able to bolt it right in, using the shock plates and u bolts from the 74. You might possibly have to play with the rubber flex hose length going to the tee fitting for the hard brake lines on the rear end housing. If you're removing the rear from the Duster, just take note of where the rubber line attached to the body on that car vs. your 71. Both should have 7260 U joints unless the Duster rear end is, like a 3.91 Sure Grip from a 340 4 speed Duster...lol. Shorten your driveshaft to the proper length and you will be good to go.

U bolts and shock plates will be different, also take note that the track width of the 8.25 is wider by roughly 3/4 inch.