I have put a pick-up body on a Blazer frame, I used a 77 frame which was a straight frame like a pick-up has and the third set back body mounts were inbd of the frame rails like a pick-up.You could probably use the 80s frame and relocate the threaded holes on the cab rear support channel.
I cut a short box down 11" right behind the first stake pocket where the tank door was and used the door in the back where the Blazer fill goes.
It used to have alot of people stratching there heads wondering because it just didnt look right for a short bed but it had a full cab back.
I have since sold the truck but I had it over ten years and it went thru many changes over the years(tires,axles,trans,springs,etc.).
Some pictures of it on pg.14 in the members rides, click on the camo Blazer.
RGF