Yep, I am pretty sure a GTO in 64 was a Nova type rear, leaf spring 42 7/16" center to center measurement The rear will be 1" narrower but should not cause any trouble with tires and install.
Only thing that will be different is the drive shaft length...The drive shaft will need a 3/4" shortening. This should be measured AFTER the rear is installed into the chassis so the exact amount will be correct needed to shorten or find the correct length.
