These trucks did have the option of a shoulder belt, but hardly any trucks had them installed, so most trucks, like yours (and mine) had a plastic cap where the bolt holes are. That's correct, the female parts were all adjustable length straps (even for the driver and passenger seats).
There's a set on ebay right now, althought the price is pretty steep. It will give you an idea of what they look like though.
http://cgi.ebay.com/ebaymotors/67-68...spagenameZWDVW