A Body Tire Clearance

A street- friendly 400 should live quite well with an 8.25 rear end- why subject yourself to all the issues involved with changing it out?
With the Dr. Diff kit installed, just figure out the backspacing you'll need on your wheels and slap 'em on.
If you need gears or a Sure-Grip for your rear, Jeep Cherokees, early Dakotas, and even some half ton trucks and vans commonly had decent gears and Sure-Grips in them, and can be sourced pretty cheaply from the boneyard.
Just be aware that the axle spline count changed in the mid/late nineties at some point, so you need to stay with the early versions.