For safety - original bench or switch to bucket?

If you have a four door, then at least the seat backs are fixed. (On the '67 and early 2 door cars , the seatbacks fold forward and have no latch)
Headrest is definately a good safety feature. You can't go wrong there. While everyone has a different sense of what is "sufficiently safe" if you feel safer driving yourself and any passenger with a headrest, then go ahead and do that. One option is aftermarket bucket seats such Flowfit, Reccaros and so forth. Another is to use a bench seat from a car that came with headrests. Headrests were an option in '67 (rare on US models) and more common starting in '68.
Oddly enough, headrests were not mandatory in trucks and multipurpose vehicles until 1985!

A shoulder belt is another option. I've seen no evidence for '67s having a provision to mount them to the roofline, so thats a little more care is needed to locate and reinforce the mounting. There is at least one company that offers three point belts for retrofitting into older cars.