I've already found one tucked up under the driver side bucket seat for my Fremont built '72 Supreme, but what's the likelihood of there being a second one above the gas tank?

 

Pull it down a little and shine a flashlight across the top of the tank. You dont have to remove it unless you find one.
There being one is all dependant on who was installing gas tanks the day your car was built

 

Would it be easy to see or underneath the padding?

 

I had an Arlington-built 71 Cutlass Supreme back in the late 1980s; found one under the rear package tray when I repainted the car, also found one in the drivers bucket seat back when I recovered the seats.

 

I dont know Fremonts build process, so all I can suggest is you drive the car till its empty,
loosen the 2 bolts that hold the straps, lower it a bit and look.
While your there, inspect the condition of your fuel and evap hoses, and replace as necessary

 

Purpose for finding a SECOND build sheet?
ya greedy bastard.

 

Well, when I found the first one, I made the hasty (and probably not the best) decision to laminate it. It's in really good shape and has held up well being tucked away in my safe. So if I find a second one, I'm going just put in a plastic sleeve and tuck it away with my other one.