In a quick review of Jerry Mac's book it looked as if some of the "C Stock class" were wearing shoulder belts. I'm guessing it was based on the speed or ETs. Jerry Mac. should be able to answer this question if he gets on-line, since his 68 Camaro was raced back and still today. D Stock Eliminator if I remember correctly and still races today.
Below is a clip from a search from the 1968 NHRA rules, its not very clear unless you dive into which class...
10:11 SEAT BELTS
All cars not required by Class Requirements to use SFI 16.1 driver
restraint systems must be equipped with an accepted quick-release type
driver seat belt. Belts must be securely fastened to the frame,
cross member, or reinforced mounting so that all fittings are in a
direct line with the direction of pull. Seat belts may not be wrapped
around lower frame rails. Steel castings of the type recommended by
FAA or U-bolt-type mounts are permitted. If used for installation, flat
steel plates must be a minimum of 1/4-inch thickness and have
rounded edges to prevent cutting seat belts. Under no
circumstances can belts be installed with bolts through webbing. In
all cars with fiberglass floors, a cross member (minimum 2-inch x 2-
inch x .083-inch wall thickness square tubing) must be installed
between frame rails for proper driverís seat-belt installation.
So, I'm voting no for D Stock and lower classed that were run back then, but I could be wrong again..