The only way I know of is to remove the original webbing and add new cloth to original metal parts. There are seatbelt reconditioning services but not sure if they are gov't certified, since the proper cloth must be properly sewn for safety. This is really a question for Simpson Racing Products or other well known maker of seat belts & harnesses.
If you're poor, try redying or even upholstery-painting the belt material you already have; note this will not work very well if the belts you have are black. Aggressive bleaches may so affect the cloth or the thread used that its function as a life-saving safety device could be compromised; after all, they may be almost 40 years old. No safety equipment lasts forever, nor was it designed to do so. Racing and military seat belts & harnesses have a date tag and are thrown away after a time, just like old helmets.