I did the same thing 66 Cuda did on my '65 Dart Charger when I installed an 8-3/4 diff. I wouldn't think the process would be any different if you fitted the springs to a 7-1/4 diff.
When I rebuilt my suspension I used a book, "Performance Handling For Classic Mopars" by Tom Condran. I highly recommend the book as a reference and how to upgrade your suspension to modern standards. On page 101 he describes how to lower the rear ride height safely without inducing oversteer. Cut two 4-1/4" pieces off 3-1/2" X 3-1/2" X 3/16" square tubing and replace the front spring hangers with them. Use the old hangers as templates for drilling new holes, and either use the old studs or new 5/16" NF studs pressed into the new hangers. Raising the holes 1" will lower the rear bumper height approximately 2-1/4", but it's not a precise adjustment. The HD springs raise the rear bumper height about 4", so lowering it 2-1/4" should be about right for me.
