building a mild Daily driver 318

Grant nailed that. An 8-1/4 is the inexpensive unit to use. It is very popular today. It is still in production today. Parts are cheap, over the counter available and newer modern parts can be swapped onto an older unit, like a Duster that a 8-1/4 came stock on.

The nose of the rear is slightly longer. All the tears do this as they go up in size. You will need to shorten the driveshaft. The other thing is you will have to ether get with the rear or source out are the shock mounting plates that go under the spring and the "U" bolts that attach the rear to the springs and shock plates under the springs are 3 inches for the axle tube. The 7-1/4 is 2-1/2 inches. All other axle tubes are 3 inches.

Sure grip units (AKA Posi for the GM guys) or spools and the ring gears are also inexpensive.
Many gear ratios exist for this rear. It's strength is very good.

You can find them here on the forum or old junk yards for approx. 2-250 bucks