Not to contradict DetMatt, but you might want to check to see when head restraints became standard and/or optional. There's all of the related bits that are buried in the seatback to accept the headrests, and I think it's a safe bet to say that Chrysler wouldn't just waste the $$ throwing that stuff into the seatbacks of every car on the hope that somebody would buy headrestraints optionally. Once they were standard, you'd be fine, but before that, be careful.
Shoulder belts "might" be a little easier. The important piece from them was the weld nut in the ceiling. Once they became optional adding them into every car wouldn't have cost a fortune, so they might have installed at 100%.
By and large most of what you're looking for became standard by the end of the 1968 run, so you might want to look for somebody's parts car from that era.
Thanks, that's great info. Here's what Allpar has to say about Chrysler cars (not sure if this would apply to Dodges, too?)
in 1967, numerous safety options became standard features. For Chrysler brand cars, these included a remote-control rear-view mirror, better front and rear seat belts, and seat belts for the third seat on wagons. Primitive shoulder belts were an option; these folded into roof clips but did not adjust automatically, and restricted movement once in place. The bolts for these belts were added to each car, whether they were ordered or not, so they could be retrofitted. Center-seat belts were optional, but again the bolts were now put into each car so they, too, could be retrofitted
http://www.allpar.com/history/chrysler-years/1966.php in the section called "Safety" near the bottom of the page