The spring should have a slight downturn below horizontal at each end (see pic); you can verify if it's in spec by measuring from the centerline of the inner and outer camber strut bushing bolts to ground, and subtracting the smaller from the larger number. The difference (Dimension "D" in UPC 4 in your Assembly Manual) should be about 3-3/8" with a full tank of fuel; that measures spring deflection, and is independent of tire size.
Unless your spring is heavily rusted/pitted, with significant material loss, removing it, disassembling, cleaning, painting, and re-assembling with new liners (paint and liners from Quanta) may bring it back in spec. Also replace the rubber bushings for the spring end link bolts - frequently they alone are responsible for low rear ride height, as they compress as they deteriorate.
There was no "big-block" spring - all '64-up midyears used the 9-leaf standard spring (three flat top leaves, the bottom six were cambered). Cars with F40 or F41 used the 7-leaf spring (all leaves cambered), and that spring is a LOT higher rate than the 9-leaf.
Most of the reproduction steel springs make the rear ride height about 1"-2" too high, although that can be overcome with the "long" adjustable spring end link bolts.
I have a new Eaton reproduction steel 9-leaf spring (
www.eatonsprings.com ) on my Top Flight '67 with stock end link bolts and new bushings, and it only sits about 1/2" high at the rear - hardly noticeable.
