First of all, thanks for your concern. I appreciate any and all opinions on any aspect of my build. I am not a know-it-all and although I have a fair bit of experience under my belt there are always ideas and concerns to weigh when modifying a car, especially one with a few more horses than was original. If we keep each other safe out there the hobby has a better reputation, and..well..we are safe.
The safety aspect, although not worrying me personally, could cause issues should a tech inspector at a track have issue with it depending how fast the car went. I do not really intend to track this car, so that is not too big a deal, and as the tank will be pretty much stock appearing, I think they would not even notice it. that being said...
The tops of these tanks are pretty tough, and do not tend to leak up through the part that forms the trunk floor. They are designed to be a trunk floor. Yes this is a hatch car, so open to the interior, I see where you are going with that, but in essence there is no difference in a 60's era trunk vs a Mustang II hatch in that neither are sealed from the occupant area anyway, with only seat fabric and fabric package tray separating the two..fuel being a liquid and a gas, will travel the same regardless of a seat in the way. Although I do not smoke, I would not build it in such a way that any fumes or rubber or liquid would leak or be accessible anyway.
I think actually a standard type fuel cell is more hazardous than this mustang tank in a hatchback installation. All the fuel cell filling and sending parts are on top, and vented to the interior unless an outside fill tube is adapted, which still leaves the sender to leak inside the car. Most popular fuel cells are plastic which also seems less safe. Granted if I went for a circle track steel and bladdered outside filled fuel cell for 700 to 1000 bucks it would probably be safe, but I am going for a different look.
I do have a possible alternate idea of mounting the 65 tank below the floor, which would require cutting out the spare tire well also and re skinning the floor. I may yet do that. The drop in mounting would work alright, but I find myself wanting to be picky on how it's mounted which would involve fabbing up a shelf lip ala' 65 mustang trunk for it to mount to neatly and correctly. As this would be a lot more work, I think I can successfully under mount it once the floor is re skinned with perhaps less fabbing. I wasn't planning on using the stock filler location anyway so filler plumbing will be a fab job anyhow. The fuel filler will exit the rear under the licence plate. I am running the stock rear battering ram bumper, so protection is as good as any car. The 65 tank is quite shallow, so It will not hang down even as much as the stock tank.
If none of it looks safe when I mock it up, I will not go with it. It's kind of hard to say at this point until it is mocked up.