My friend (a full-time, self-employed mustang restorer) custom installed front and rear shoulder harnesses in a customer's 65 coupe. He used seatbelts from an '84 mustang and fed the front shoulder harnesses down through the top of the inside quarter trim to an anchor he fabbed at the bottom. With the plastic ferrel from the '84 installed to guide it through the top of the panel and another steel reinforcement flush to the inside of the panel, the whole system looked factory. The rear harnesses were installed much the same with the straps being guided through reinforcements under the package tray and all the reels were out of vision. I saw the install at various stages through the project and there didn't appear to be any magic involved.