My 67 had drums all around when I bought it. Had to stand on the pedal to stop. I had Wilwood calipers and rotors installed along with a power brake booster and master cylinder. World of difference. I will probably add rear discs sometime down the rotor just for eye appeal. A few years ago I put Firebird rear discs on my S10 Blazer and that made a significant difference in stopping. I dooubt that would be the case with our Mustangs.
You said it all... rear disk conversions for a weekend cruiser are hard to get your moneys worth. I started with drums all around and swapped out the front-end with disks. That made an incredible difference with-out a doubt. I would lover to upgrade to disks all around, but have other areas where the money will be better spent now (IMHO).