The '68-'73 separate shoulder harnesses are a damnuisance and their safety performance is not very good due to the poor mounting point. They were designed for minimal/cheapest compliance with the new-for-'68 requirement that they be installed, knowing nobody was going to use them. Can't recommend retrofitting these to a car like your '67 (which no, does not have the holes).
I used to recommend retrofitting 4-point Wesco belts as I did on my '71, but
not any more (the article is correct in its facts and physics, and everything said about 5-point harnesses is equally true of the 4-point variety, except the injuries that a 5-point can do to your 'nads).
When I get rid of the separate shoulder straps in my '73, I'm putting in
these up-to-date, reputable-brand ECE-approved 3-point seat belts (linked vendor is reliable; I've been buying there for a decade). The 514-30 can be installed in cars without a B-pillar (such as a Barracuda). More info on mounting at the
maker's website. If I needed mounting hardware, I'd buy it again
from Wesco, but the Securon belts are a much better product than anything Wesco carries.
