It is possible the mount locations are different and the blocks are not, but as I think about this the change in frame profile may mean both are different after '53.
I've very skeptical that they changed the Suburban/panel body behind the doors when they changed the front and the bed. But someone is going to need to crawl around inside and under a couple of them to take some measurements unless we can access a '53 or earlier Assembly Manual.
I have both a '53 (parts) and a '55 1st GMC Suburban at home but I'm also currently disabled to the point I'm using a walker to get around and there is no way I'm getting down on a creeper to measure any of this. It may be months before that changes.
I've made some rash assumptions regarding the body parts and have cut up two earlier panel trucks because I believe the body parts are the same once you get behind the doors. One of them was a '48 the other was a '52 or '53.
I believe the answer to the original poster's question is yes it can be done. A '54 pick up box on an earlier frame no, but a panel or Suburban, yes.
