Hi Joe ;
WELCOME ! 
You'll find a folding back seat in some old junker , so check EVERY ONE at the local Junk Yards as many have been changed .
'67 does not have it from new as far as I can tell .
I have a shattered spine , fused neck and broken skull so I understand your pain needs .
I use black foam from the Furniture Upholstery Supply house , the Auto Trimmers simply cannot get real firm foam , I don't know why not .
ALWAYS lay burlap or muslin over the bare springs before adding the hard foam or cotton padding , then the cover .
How about some photos of your truck ? . doesn't have to be shiny or even running .