Re: 66 chevelle body mounts
I just researched that empty slot, before the rear springs and discovered that is only populated if its a Wagon /or Pick UP, frames are pre-drilled for a variety of applications.
On 66 hardtop there are 12, 6 on each side including the radiator support. (no bushing before spring)
On 66 convertible there are 14, 7 on each side with an extra mount inner frame in back of front wheel.including the radiator support.
On 66 Wagon (never seen these) 16, 7, including the radiator support, Bushing in front of rear spring is populated.