I never put a '68 seat in a 73 up GM truck,but I've adapted plenty of others into them...
I had a bench seat from a '68 Chevelle in a 74 C-10 years ago...seat was mint,my trucks was tattered,hated to see it tossed into a dumpster when a body shop changed the interior to bucket seats..it took a little fabrication to mount it,but it fit pretty good..
I'd do as suggested--first measure & see if the '68 seat is not so wide it wont let the doors shut!,or stick out too close to the dash....I'd guess its probably close to the '82 seats width and depth..
The brackets will be different on the '68 seat,the floors are a lot different than the 73 and up trucks--you may be able to take the ones off the '82 seat and put them on the older seat provided you have them..that may require drilling and bolting or welding them on.
Bolt the brackets to the floor,plop the seat on them,center it,see if you can get away with marking the bolt holes on the seat frame and drill & bolt them on..
You might get lucky,that might be all you have to do..only one way to find out...taking the seat out can be "fun" if the bolts have not been removed since new though...they may break,and have to be drilled out..but there is only 6 bolts I think..
I used a bench seat out of a '70's Caddiliac that was leather and 6 way power in my '72 K5..it was a tight fit,it touched the door panels slightly,but man,it was like driving a couch down the road with that seat!..
I also took the bench seat tracks off the bench seat on a '77 GMC I had and bolted them back to the floor,then bolted angle iron across them so I could bolt bucket seats in it..they both moved when you went to adjust then though,not separately...they were a bit too high for me ,I had to put a smaller steering wheel on to miss my legs..