As you already know, '71-'72 Panteras use a separately attachable shoulder strap into the stock lap belt fastener. So to add a retractor would mean replacement of the entire seat belt system with the later style and adding big holes etc. All that usually goes wrong with the stock early system is, the plastic decoration on the buckle stress-cracks from 50 years of internal spring pressure. This then allows the whole buckle to spontaneously disintegrate. Larry Stock's Pantera Parts in NV rebuilds failed stock buckles on an exchange basis.
Another nice-to-have mod is a stiffener system on the short buckle-end like the Pantera Gr-3, so the buckle stays erect and can be conveniently attached one-handed. This can either be a stiff plastic outside sleeve or a wire type stiffener. There was an illustrated POCA article decades ago on doing this. For badly damaged cloth belts, the jump-seat belts from mid-'80s Ford crew-cab pickups are about the correct length (but have chrome buckles), or there are restoration shops that re-web OEM belts. Remember, all this concerns SAFETY SYSTEMS on a 160 mph car! Do it right!