I bought a bench a couple of years ago but have not installed it fully yet which is to say it's been put in my rig for a test fit but then taken out for recovering.
There's no foam in the original bench seat (mine had the factory cover still on it). I found this out when I took the covers off mine which explains why both of us couldn't find foam.
The seat is built from burlap reinfoced with wire mesh over the springs and a thick cotten pad on top of that along with some wire here and there to give it support followed by the seat cover.
For your 68 it should be a direct fit. just remove the buckets and bolt in teh bench. In my 74 I had to drill a couple of mounting holes in the floor.
If you go original bench, you can find them out on e-bay and sometimes here in the classifieds (where I bought mine). The best way to ship it is greyhound. Should be $100 to about anywhere in the lower US. Bronco Hut sells the original covers for $400 or so. I want the original look but may that price is crazy high to me. I found no place selling the correct seat springs. Mine has a couple that were broken so we made do with other springs.
If you want another option, I'm told the seats out of some Ranger pickups will fit but are not direct bolt ins. I'm sure others will add to the Ranger info.
My bench is stripped, blasted, primered and painted. Just can't seem to ge tthe budget right for the recover.....
Tim