From what I recall from seeing '63 - '66 trunks, the gap between the masonite and the trunk floor is pretty normal. Remember this trunk pan stamping was designed for tires for 6cylinder cars. So I believe a 6.50-13 will give you a better fit, but V8 cars got wider tires which will sit taller in the tire well. The V8 engine A-body car wasn't considered when approval for these panels was signed off.
