These trucks did have an optional shoulder belt... I'm not sure what year it was first offered though.
It was a total manual deal, no retraction... and it had two buckles, one for the lap, one for the shoulder part.
UPDATE: on the factory 3 point shoulder belt the top belt portion did retract to the B pillar, but the lap belt portion did not.
The 1966 trucks have a threaded bolt hole up in the corner of the cab to use these manual shoulder belts.
a factory illustration of the manual shoulder belts :
http://www.flickr.com/photos/4-5-6chevytrucks/1887238126/in/set-72157611649585198/
