Need some help 1965 barracuda fuel tank

Lee R. is correct. 64-5-6 A bodies used one strap from side to side. 67 did also but the 67 straps wont fit (to long). Check your frame rails underneath, you should be able to see where the "T" on the strap fits in the rail. Opposite that is where the "J" hook goes. You need "one strap" for an early A, and one "J" hook.
Don't forget the insulator pad/mat, that goes between the tank, and the floor board. You can make one from a home door mat.
Lee's strap looks correct.
Dave