Your requirements pretty much restrict you to what you already have. The two-piston calipers shown in above posts are Gr-3 or Mangusta rear calipers, and pads for the pair-of-pliers style e-brake are hard to find; some say impossible. Girling made a number of these for other brands- FIAT, Aston-Martin, Rolls-Royce etc. GT-5 and GT-5S Panteras used a larger variant of the RR brake with a 1.25" thick vented rotors and enormous 3-piston iron calipers.
A few owners use two complete stock rear calipers on each stock upright, hooking up only one e-brake per side. This provides a stock appearing, bolt-on brake system with legal e-brake that is better than pure-stock but extremely heavy. The stock non-adjustable proportioning valve will not be balanced with 4 rear calipers (actually, its not balanced with stock brakes and modern tires, either!) so should be discarded and brake plumbing reconfigured.
An option requiring machining is to adapt a Wilwood two-shoe drum e-brake that is built into the hat-section of the stock rotor. Early Corvettes also use such things. While not at all 'stock', the parts are completely hidden inside a modified rotor except for the actuating cable.
Stock '71-'76 Panteras had no separate 'hats' on their solid 0.875"-thick rotors. Vented 11.5" OD rotors from early 911 Porsches or some Volvos, with custom built spacers, will fit and markedly reduce weight; paint aluminum spacers black for a 'stock' look. There are some vented one piece late-'60s Mustang rotors - with no 'hats'- that bolt on but are extremely hard to find, as two-piece rotors supplanted the one-piece design several decades ago. Brake mods are serious business! There are a large number of pertinate articles in the POCA Club Archives on brake improvement (membership req'd for access).