autoxcuda would be the guy to talk to. That having been said, I'm pretty sure he's said in the past that the 10" drums are fine in the back and that most of the autoX guys just run those. 11" drums are significantly heavier, and not necessary on the lighter a bodies.
My Challenger is set up in a similar manner (agressive road handling), I have 11.75" rotors on the front, I have a set of 11" drums for the back (heavier car already), and 1.12" torsion bars with tubular UCA's and LCA's. I'm not done yet, still need shocks and sway bars. But its getting there. The larger torsion bars and 11.75" rotors make a HUGE difference in the handling of these cars.
But if the guys that have rear disks say they didn't improve their stopping distance, why bother? There are only a couple of possible advantages. Disks tend to have more of their surface exposed to moving air, so they MIGHT cool faster. And the pads are easier to change with less maintenance to keep them adjusted properly, which is obviously a big deal. Weight-wise I'm not sure they're any lighter, since you have to consider both the rotor and the caliper. They might even be heavier if you're running OEM calipers. And I really prefer the adjustable style rear bearings, especially vs green bearings, which are lousy at supporting cornering loads. The type that uses a Ford 9" style bearing might be better, but it depends on WHICH ford 9" rear axle bearing is used, Ford changed this several times over the years, some are a good design, some are not.
If nothing else, $500 is a lot of money to spend for a part that effects only 20-30% of your braking force. You could spend much less than that on a set of 11 3/4" front rotors, caliper brackets and '73 up spindles (if you don't have them already) and see a marked improvement in stopping distance and power, since your improving a part that controls 70-80% of your braking.
