For the brakes, consider pressure and booster requirements. I run the Cobra style on a '70E with EBC yellowstuff pads... works great and needed a double 8" booster and 15/16" master for adequate line pressure. I don't know piston area on the Brembos. It all fits under 17" wheels.

As far as rear springs, in round numbers stock low po are 120#/inch, MOPAR XHD are 140#/inch and XHD+full added leaf are 170#/in. I'm running the XHD+1 leaf. The added leaf corrected the sag as the springs aged and got the rate where I wanted it.