Good find, having the voltage that low will make the pump not put out the proper fuel flow.
Kenne Bell, has a Boost a pump, that you can increase voltage as boost increases, even if your alternator fails. I have one, I just bought it for my Syclone.
I would guess the back spacing would be 5". I had a 1965 SS Malibu ,I sold it to my friend. Good luck!
