I have both front and back seatbelts in my '65 because I want to take the grandkids riding. The law is irrelevant to me; safety is the concern. Keeping those kids secured. You have to drill for the rears, because they weren't offered in the Mustang, even as an option. Be sure to use a proper kit with big heavy washers, to spread the load, as the sheet metal is thin there.
As for the exploding gas tank, that's mainly bravo sierra. A few Pintos had the problem but that's about it. I was rearended hard enough (years ago) to knock me 61 feet, buckle the frame and split the tank but no gas got inside. All on the ground. And no fire. Not in a Mustang - but in a Ford with a similar tank setup. (As for 60 Minutes, does anybody remember the pickups with exploding gas tanks a couple of years ago? Wasn't that 60 Minutes who was using explosive charges to make them blow, because they couldn't get them to do what they said they did??)