As a minimum I think you need to do the following....
First (this is required) get some good brakes. I would go with the Granada swap. Its cheap and reliable. You don't need rear disks, they don't do much of the stopping anyway.
First and half, make sure that all the suspension is rebuilt or in good shape.
Second, 3 point belts and high back buckets from a modern Mustang.
Third, instead of a fuel cell, you may consider a metal trunk separation plate. At leat when you get rear ended, you don't have that rush of fuel heading at you. Its much cheaper, but certainly no where near the safety of a fuel cell. But no commercially available car has a fuel cell either.
Fourth, make the engine a little more reliable. Get a professionally rebuild carb with electric choke and change out the points for a Pertronix Igniter.
Then, A/C if you want it, a radio, etc.
