Yes, you need to remove the 4 bolts that hold the engine in place and move it forward an inch or two. Or, you can do it by splitting the tractor. Which, if you are exchanging rear ends, you will be doing.
Two of the engine bolts are above the front axle, so you need to jack up the front and let the axle pivot right and left to reach those bolts.
If it's just a bent axle, you can just change out the axle. Open the rear axle cover, pull the big C clip from the end of an axle, and pull it out. Do the same on your 71, then put the donor axle in. Change fluid and rear plate gasket, refill with Hytran or similar. Replace the rear axle housing bearing and seal on that side if you think they may have been damaged, cheap enough while you have the axle out.
Let's see some pics of your dually 71!
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