I put a three point set in the front of my 66 hardtop over ten years ago, same scenerio - women. It's not difficult but that installation required additional spacers at the floor and a large plate for stress under the car for the retractor. At the upper rear of the door frame is a "roller" for the belt to be drawn over the driver/passenger to the center "hump". That requires a 7/16 inch hole and a couple of pop rivets. At the center, if you have a console, you might need an extender for that short section of belt. Somewhere I've got pictures of the "roller" installation but I got to filter thru several hundred to find them. It cost me about $225 US and the price seems to be the same today. You can find almost everything you need from newer cars except the "roller" section. With a convertible's additional body bracing at the floor, the retractor will be a couple of inches further in than my hardtop but should not overly restrict access to the rear seat.