I really don't think your going to be sure of anything until you take the rear rotors off and do a visual of the emergency shoes and the rotors. When you converted to power brakes did they recommend changing the proportioning valve ? Calipers don't adjust like drum brakes do. The pistons, just naturally due to gravity and the fact each piston has a spring behind it keeping outward pressure, will extend as far as possible until something stops them, when the pad hits the rotor. If a piston is cocked or pitted sometimes it becomes stuck in the caliper and when the pedal is stepped on and released it doesen't release properly thus causing a drag. Usually when this happens you will feel a pull when you step on the brake. Care must be taken when you remove a caliper from the rotor as the pistons will push outward until the dust boot stops it. They make special clips that you insert into the caliper before removal to prevent this. I use to just wire tie the pads to the caliper before removal to prevent this.
The rear brake and suspension system on your car is very unique, but the technology is 40 plus years old. As I said before if your rear brakes were smoking something got REAL HOT. More than likely it was the emergency brake shoes. You need to remove the wheels, take off the calipers and rotors and inspect the shoes for wear and cracking, check the drums for hot spots, you'll know cause they will look kinda blue. (As I said before if the shoes are extended rotor removal will be a hassle as the rotor will not just slip off at that point.) You need to check the hardware, the little brake return springs as they could have become fatigued from heat and take apart the adjusters at the bottom of the shoes and grease them to make sure they spin freely and adjust. Once you do all this put em back together, adjust the shoes until they just start to drag and you should be all set. If you have never done a brake job on drum brakes perhaps you should have someone who has experience do it as you don't want brake failure as you have a pretty big investment going there.
I also mentioned about checking the rear wheel bearings for play. You should make sure you do this. What happens is the rear wheel bearings wear at an angle. The pressure from the tire being on the ground now changes and causes the bearing to heat up where it rides on the spindle. Eventually it gets so hot the spindle snaps in half, the car drops to the ground and the wheel goes thru the top of the fender and the car comes to a grinding hault riding on the trailing arm. It's very easy to check. Get the wheel off the ground, place your right hand on the tire at 6 o'clock, fingers on the tread, your left hand at 12:00 o'clock, fingers on the tread, and then gently try to rock the tire. Do the same at 3 and 9. The tire shouldn't move or rock at all. If it does you might have bearing issues that need to be addressed.
If you check the cables to make sure they move freely, inspect, clean and adjust the emergency brake shoes and make sure their operation is functional, check all the involved components for wear and tear and assemble all the parts correctly you will eliminate immensly the chance for a failure and insure yourself that all is functioning as intended and eliminate them as the source of your problem.
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